text:
Rays of Positive Electricity and Their Application to Chemical Analyses/Retrograde and Anode Rays

The rays we have hitherto been considering consist of positively charged particles travelling in the direction in which such particles would be moved by the electric field in the discharge tube. In addition to these there is another system of rays travelling in the opposite direction. By far the larger portion of these rays are cathode rays, i.e. negatively charged corpuscles moving with great velocity, but as the author showed long ago$1$ these are mixed with rays which are evidently of a different character, for unlike the cathode rays they are not appreciably deflected when a permanent magnet is brought near them. It was afterwards shown by Villard$2$ and the author$3$ that some of these new rays were deflected by strong electric and magnetic fields and that the direction of the deflection indicated that the particles forming the rays were charged with positive electricity. The fact that these rays travel with high velocities away from the cathode and thus in the opposite direction to the electric forces acting upon them makes their investigation a matter of very considerable interest The apparatus I have used for this purpose is represented in Fig. 40.

A is a perforated electrode through which the rays pass on their way to the willemite screen or photographic plate S. On their journey to S the rays traverse the usual electric and magnetic fields. B is a plane rectangular electrode at the other end of the discharge tube: it is carried by a stopper working in a ground glass joint and thus can be rotated about a vertical axis. C is a wire fused in the side of the tube for use as an auxiliary electrode. D is a side tube in which a closed glass vessel containing a piece of iron can slide up or down: this vessel carries a piece of fine metal rod which, by moving the iron by means of a magnet, can be inserted in or withdrawn from the line of fire of particles projected from B. When the stopper carrying the electrode B is turned so that the normal of the plane of the electrode either coincides with the axis of the hole through A, or makes but a small angle with it, then if B Is made cathode and a discharge sent through the tube, the cathode rays pass down through the tube in A and produce vivid phosphorescence on the screen. In addition to these rays there are others which produce a phosphorescence different In colour from that due to the cathode rays and are deflected in the opposite direction by the and the magnetic fields: the amount of electrostatic deflection is about the same as that for the cathode rays but the magnetic deflection is very much less. It can easily be shown that these are not ordinary positive rays due to A becoming cathode through accidental reversals of the coil. For in the first place they disappear when the electrode B Is twisted round so that a normal to its plane no longer nearly passes down the tube through A: and secondly the rays persist when A is disconnected from the induction coil and the auxiliary electrode C used as a cathode. Again when the rod attached to D Is put in the line of fire a shadow is thrown on the phosphorescence on the screen, due to these rays. These rays are strongest when the electrode B is placed so as to be at right angles to the axis of the tube through A. If the electrode Is rotated they diminish rapidly In intensity but can be detected until the normal to B make an angle of about 159 with the axis of the tube through A; they appear in fact to follow much the same path as the cathode rays from B, for much the same rotation was required to prevent the cathode rays getting through the tube In A and producing phosphorescence on the screen.

These rays get exceedingly feeble when the pressure of the gas in the discharge tube Is very low and they are no longer observable at pressures when the ordinary positive rays give quite vigorous effects; even when most fully developed they are feeble In comparison with the ordinary positive rays, so that It Is necessary for the tube through A to have a much wider bore than Is required for experiments with positive rays. As these rays travel In the opposite direction to the positive rays they are called retro-grade rays.

Using a tube through A about .5 mm. in diameter I obtained a photograph of the retrograde rays which gave the following results:—

There are in the retrograde rays positively electrified atoms and molecules of hydrogen and positively electrified atoms of oxygen: there are also negatively electrified atoms of hydrogen and oxygen, and with these rays the intensity of the lines corresponding to the negatively electrified particles is greater than that of the positively electrified ones; with the ordinary positive rays the positive lines are much stronger than the negative. In. the retrograde as well as in the positive rays are large numbers of uncharged particles. The photograph taken with the retrograde rays shows that the maximum velocity of the negatively electrified atom is about the same as that of the corresponding positively electrified one and differs but little from the velocity of these atoms in the ordinary positive rays. This result is suggestive because the electric field in the tube would accelerate the negatively electrified retrograde rays and retard the positively electrified one. It points, I think, to the conclusion that the origin of the retrograde rays is analogous to that of the negatively electrified particles which accompany the positive rays, the difference between them being that the retrograde rays acquire their negative charge before passing through the cathode, while the negative constituent of the positive rays do so after passing through the cathode. We may suppose that the process by which the retrograde rays are produced is somewhat as follows : neutral atoms or molecules acquire a negative charge when they are just in front of the cathode, they are then repelled from the cathode and driven through the dark space, acquiring under the electric field in the discharge tube a velocity of the same order as that acquired by the positively electrified particles of the positive rays during their approach to the cathode. Some of these rapidly moving negatively electrified particles will in their course through the gas come Into collision with the corpuscles and molecules In the discharge tubes; the first collision will detach a corpuscle leaving the particle in the neutral condition; another collision will detach another corpuscle and leave the particle positively charged The particles which have made two collisions form the positively electrified portion of the retrograde rays, those which have made one collision the  portion   which   is   without  charge,   and which  have not made a collision the  negatively electrified portion of these rays.

These retrograde rays are very well developed when a double cathode of the kind introduced by Goldstein (see p. 5) is used instead of a flat cathode. If a cathode consisting of two parallel triangular plates. Fig. 41, is substituted for the flat cathode B in the apparatus, shown in Fig. 40, a plentiful supply of retrograde rays come from the cathode when it is turned into a suitable position. By twisting the triangle round by means of the glass stopper the emission of the rays, both cathodic and retrograde, could be determined. In this way it was shown that the maximum emission of cathodic rays is along the line starting from the middle points of the sides. At the higher pressures this is practically the only direction in which cathode rays can be detected; at very low pressures, however, cathode rays can be detected coming from the corners of the triangle as well as from the middle points of the sides. Few, if any, however, are given out in any intermediate direction. The positively electrified particles stream off at all pressures from both the corners and middle points of the sides, but not from the intermediate positions. The most abundant stream comes, as for the cathode rays, from the middle points of the sides but the disproportion between the streams from the corners and from the middle points of the sides is nothing like so large as for the cathode rays, so that the ratio of positive to cathode rays is much the greatest at the corners of the triangle.

A simple method of demonstrating the existence of retrograde rays and also of the places at which the positive rays originate Is founded on the difference between the phosphorescence of lithium chloride under cathode and positive rays. When lithium chloride is struck by cathode rays, the phosphorescence is a steely blue giving a continuous spectrum. When struck by rapidly moving positively electrified particles the phosphorescence is a rich deep red, and the red lithium line is very bright in the spectrum. To explore the tube for positive rays a thin rectangular strip of mica or metal Is covered with fused lithium chloride, the strip Is attached to a piece of iron$4$ which rests on the bottom of the discharge tube. By moving the iron by means of a magnet the strip can be moved towards the cathode or away from it. If we start with the mica strip close to the cathode we find that there Is no red light to be seen on the side of the lithium chloride next the cathode. The anode side of the chloride Is a brilliant red, showing that the strip Is being struck by the positive rays before they reach the cathode but not by the retrograde ones. If the mica strip is pulled farther away from the cathode until the distance between them is about half the thickness of the dark space, red light appears upon both sides of the strip showing that now it is struck by the retrograde as well as by the positive rays. This state of things continues until the mica reaches the limit of the dark space and approaches the negative glow; In this position the cathode side of the strip Is red but the other side Isdark showing that now it is struck only by the retrograde rays. Another way of making this experiment is to keep the strip fixed at a distance of between one or two centimetres from the cathode. Beginning with a fairly high pressure so that the strip is outside the dark space, we find that the cathode side of the strip is reds while the other side is dark; in this position the strip is struck only by the retrograde rays. If the pressure is gradually reduced so that the dark space increases until it reaches just past the mlcas both sides of the strips will now show the red light, showing that now positive as well as retrograde rays strike the strip. When the pressure is further reduced until the dark space is three or four centimetres long, the red light disappears from the cathode side but is very bright on the other.

These experiments show that many of the positive rays start from close to the junction of the dark space and the negative glow. It is surprising to find how short is the distance which the screen has to travel from the negative glow before the redness of the side remote from the cathode shows that positive rays are striking against it. As at this end of the dark space the electric force is very feeble, the charged particle can only have fallen through a small fraction of the potential difference between the anode and cathode; yet as we have seen it has the power of exciting the lithium red light. The reason that in the preceding experiment the retrograde rays are not observed when the screen is close to the cathode is due I think to the shadow cast by the mica on the cathode. The mica stops the positive rays on their way to the cathode so that the parts in shadow are not struck by these rays and so cannot be the origin of retrograde rays, if these are produced in the way we have described.

This view is confirmed by the following experiments. If the cathode is placed near the middle of a large bulb and the mica screen is put a little on one side of the cathode, the red lithium light can be observed on the side of the screen turned towards the cathode even when the screen is quite close to the cathode and the dark space 5 or 6 cm. long.

Again if the cathode stretches across a tube of uniform bore, and the screen is moved towards the cathode, the shadow thrown on the cathode becomes much more marked and suddenly increases in size at the place where the red light fades away from the cathode side of the mica strip. The increase in size is due I think to the screen getting positively electrified when in the region close to the cathode. We know by the distribution of electric force In the dark space that there is a dense accumulation of positive electricity just in front of the cathode, which naturally would charge up an insulator placed within it. The positively electrified screen repels the positively electrified particles which pass it on their way to the cathode and deflects them from their course, so that they strike the cathode beyond the projection on it of the screen. In this way a considerably Increased area is screened from the impact of the positively electrified particles. The portion so screened no longer emits cathode rays. Thus the region in front of it is traversed by little if any current and there is consequently no bombardment of the screen by retrograde rays.

Somewhat similar effects are obtained if the mica screen Is replaced by a very fine platinum wire. If this wire is slowly moved, towards the cathode, starting from a place inside the negative glow, the following effects are observed: almost immediately after entering the dark space the wire becomes red hot and remains so until it reaches the velvety glow immediately in front of the cathode (known as Goldstein's first layer). Here It becomes cold and the shadow which before could hardly be detected now becomes well marked, and much thicker than the wire. The change takes place very abruptly. In some cases just before entering this layer the shadow is reversed, i.e. the projection of the wire on the cathode is now brighter than the rest of the cathode, indicating I think that the wire when In this position gets negatively electrified and attracts the positively electrlied particles Instead of repelling them.

The retrograde rays are well developed with cathodes made of wire gauze.

$1$ J. J. Thomson, " Proc. Camb. Phil. Soc.," IX, p. 243.

$2$ "Comptes Rendus," CXLIII, p. 673, 1906.

$3$ J. J. Thomson, " Phil. Mag,," XIV, p. 359, 1907.

$4$ It is better to put the iron in a closed tube and attach the mica strip to the tube, otherwise so much gas is given out by the iron that it is difficult to reduce the pressure sufficiently.


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Rays of Positive Electricity and Their Application to Chemical Analyses/Anode Rays

The positively charged particles, which we have hitherto considered, originate in the neighbourhood of the cathode. Gehrcke and Reichenheim$1$ have discovered rays of positively charged particles which start from the anode. Their attention was called to these rays by noticing that a pencil of yellow light streamed from a point on the anode of a tube with which they were working. It was found that there had been a speck of sodium chloride at the points on the anode from which the pencil started. They got these rays developed to a much greater extent when they used for the anode a piece of platinum foil with a little pocket in which various salts could be placed. The foil was in circuit with a battery insulated from the one used to send the current through the discharge tube; this battery was for the purpose of raising the anode to a red heat, as these rays are not developed unless this electrode is at a high temperature. The current through the tube was produced by a battery giving a potential difference of about 300 volts which, as a Wehnelt cathode was used, was sufficient to send a very considerable current through the tube: the pressure in the tube was very low. The rays were well developed in this tube when NaCl, LiCl, KCl and the chlorides of Cu, Sr, Ba, In, were placed in the pocket. The colour of the rays corresponded with the colour given to flames by the salt They did not get any effects when the oxides of calcium or barium were put in the pocket; these oxides are known when hot to give out large streams of negatively electrified corpuscles and for this reason are used for Wehnelt cathodes. These rays are apparently only given out by the salts of the metals and not by the metals themselves ; they are called Anode rays.

Gehrcke and Reichenheim arranged a Faraday cylinder so that the rays could fall into it; they found that when the rays entered the cylinder It acquired a strong positive charge.

Gehrcke and Reichenheim subsequently used another form of apparatus which gave better results than the one just described. The anode was a rod of salt placed inside a glass tube so that only the front of it was exposed to the tube; the cathode was an aluminium ring encircling the anode, the pressure was reduced to a very small value by the use of carbon cooled by liquid air. With the discharge from a powerful induction coil, or still better from a large electrostatic induction machine, the anode got hot without the aid of an auxiliary heating current, and a bright stream of rays came from the end of the salt anode; the appearance of this beam is represented In Fig. 43. It was found that a mixture of two or more salts with powdered graphite gave brighter rays than a simple salt, the best mixture seemed to be LiBr, Lil, Nal and graphite. The rays come off at right angles to the surface of the salt; thus if the surface is cut off, as in Fig. 44, the rays come off In the direction ab.

Gehrcke and Reichenheim found that there was a very considerable difference of potential between the surface of the anode and a point a centimetre or two away: in some of their experiments it was as much as 2300 volts. By assuming that the energy acquired by the rays was due to the fall through this potential V, and measuring the radius of the circle into which the rays were bent by a strong magnetic field H, the values of v and m/e can be determined, for we have

 * $$\tfrac{1}{2}mv^2 = V e$$

and if r is the radius of the circle Into which the rays are bent by a magnetic force H at right angles to their path

 * $$\frac{mv^2 }{r} = H e v$$

hence

 * $$v = \frac{2V}{Hr} $$

and

 * $$\frac{e}{m} = \frac{2V}{H^2r^2} $$

In this way the following values were obtained :—

The results for LiCI given in the first line relate to the brightest part of the rays, those In the second to the least deflected rays. It would appear from this that the charged particles are the atoms of the metal in the salt and that in the case of strontium they carry a double charge, A very Interesting case of these anode rays is that of a discharge tube with a constriction In the middle. When two bulbs A and B, about 10 cm. in diameter, with the anode in A and the cathode in B, are connected by a narrow tube: then when the pressure in the tube Is very low and a small quantity of iodine vapour is introduced into it, anode rays start from the constriction c at the cathode end of the narrow tube and cathode rays from d, the anode end of this tube. If the connecting tube were quite straight these anode rays might be the positive rays corresponding to the cathode c, but as they appear when the tube is bent this cannot be their origin. It is especially to be noticed that the anode rays do not appear unless iodine, bromine, or chlorine is in the tube. This is perhaps due to the fact that the atoms of these substances are excellent traps for negatively electrified corpuscles as they hold these corpuscles imprisoned. Any positively electrified particles in the tube will thus have a much better chance to escape being neutralized by these corpuscles when these gases are present than when they are absent: and thus the number of anode rays will be increased.

The most natural explanation of these rays is that the hot salts from which they originate act like fused electrolytes, and that the current through them into the discharge tube is carried by the ions into which the salts dissociate, the positive ion, which is a charged atom of the metallic constituent of the salt, following the current will come to the surface of the anode, will get detached from it, and under the influence of the strong electric field which exists in the gas close to the anode will acquire the high velocity characteristic of the anode rays.

$7$ "Verh. D. Phys. Gesell.," 8, p. 559; 9, pp. 76, 200, 376; 10, p. 217.

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Rays of Positive Electricity and Their Application to Chemical Analyses/Doppler Effect Shown by the Positive Rays

Before the methods described In the earlier part of this book had been fully developed. Stark$1$ had discovered a property of the positive rays which Is of great Importance In connexion with the origin of spectra, and Incidentally has led to results which have confirmed some of those obtained by the newer methods.

Stark's discovery resulted from the spectroscopic examination of the light produced by the positive rays passing through a gas at a pressure comparable with .1 mm. of mercury, a very much higher pressure than that used In the majority of the experiments with the photographic plate and the willemite screen. The stream of rays passing through a perforated cathode produces at such pressures in the gas behind the cathode considerable luminosity. Stark examined this luminosity when the gas was hydrogen with a spectroscope: (1) when the line of sight was at right angles to the direction of the rays. (2) When the line of sight was approximately In the direction of the rays. In the first case he found that the series lines for hydrogen were in their normal positions. In the second, however, he found that though there were lines In the normal positions, these lines were broadened out towards the violet end of the spectrum when the positive particles were approaching the spectroscope and towards the red end when they were receding away from it, indicating that some though not all of the systems emitting these lines were moving in the direction of the rays with velocities sufficient to give an appreciable Döppler effect. A closer examination of these lines brought out some interesting details which are illustrated In Fig. 45, Plate IV., taken from a photograph by Stark of the hydrogen line Hγ. It will be noticed that though the displaced line is broadened out into a band, this band does not begin at the undisplaced position of the line, but is separated from it by a finite distance. The alteration Δλ in the wave length λ of a line given out by a source moving towards the observer with a velocity v is by Doppler's principle given by the equation

 * $$\frac{\Delta\lambda}{\lambda} = \frac{v}{c} .$$

where c is the velocity of light. In the case of these small displacements we may take, where we are dealing with one line in the spectrum, Δλ as proportional to the displacement of the line, and we may also use this equation to determine v the velocity of the particle emitting the line. The fact that the fine line is displaced into a broad band shows that these velocities range over somewhat widely separated limits: this is quite in accordance with the results indicated by the photographs of the positive rays when deflected by electric arid magnetic forces. We saw that the parabolic arcs were of considerable length, and therefore were produced by particles moving with a wide range of velocities. The dark space between the undisplaced line and the band indicates that the moving particles do not give out the lines unless the velocity exceeds a certain value. According to Stark and Steubing$2$ this limiting velocity varies with the different lines of the same element, increasing as the wave length diminishes. The limiting velocity given by these observers for the hydrogen lines are as follows: —

 * H$α$= 1.07 x 10$7$ cm./sec. H$β$ = 1.26 x10$7$ cm, /sec.

These values are approximately proportional to the square root of the frequency of the lines. There is some difference of opinion as to whether this limiting velocity does or does not depend upon the frequency of the light Paschen$3$ came to the conclusion that it was the same for all the hydrogen lines. This velocity is small compared with the average velocity of the positive rays of hydrogen; it corresponds to a fall through a potential difference of less than 100 volts. It Is comparable In value with that which the mercury atom acquired In many of the experiments represented by the preceding photographs, when It had possessed one but only one charge throughout Its journey through the discharge tube. The maximum displacement of the line depends to some extent on the potential difference between the terminals of the discharge tube; but It does not increase nearly so quickly as the square root of that potential difference, as we should expect If even the most rapidly moving particles could give out the line: the relation between the displacement and the potential difference Is given In the following table due to Stark and Steubing.$4$ In this table r is the ratio of the kinetic energy of a particle moving with a velocity v calculated by the Doppler formula (p. 90) to the kinetic energy the particle would possess if It fell when carrying one charge through the potential difference between the terminals of the discharge tube.

 * {| style="text-align: center;width: 200px;"

! Potential Difference in Volts. ! r.
 * 390
 * .907
 * 555
 * .824
 * 600
 * .716
 * 1200
 * .622
 * 3000
 * .358
 * 4000
 * .309
 * 4000
 * .402
 * 7000
 * .274
 * }
 * .309
 * 4000
 * .402
 * 7000
 * .274
 * }
 * .274
 * }

Stark indeed suggests that his observations are compatible with the view that the deflections approach a limit corresponding to a velocity about 1.5 x 10$8$ cm./sec. and do not exceed this however large the potential difference between the terminals in the discharge tube may be. The distribution of intensity in the displaced line is very complicated and seems to be affected by the purity of the gas as well as by the potential difference between the terminals in the discharge tube. Paschen$5$ was the first to observe that there are in some cases two maxima of intensity in the displaced line, and this has been confirmed by the experiments of Stark and Steubing$6$ and of Strasser.$7$ The distribution of energy determined by Hartmann's microphotometer of the H$γ$ line in very pure hydrogen is shown in Fig. 46, taken from Strasser's paper. The first peak represents the intensity of the undeflected line, the other two the intensities of the deflected. Gehrcke and Reichenheim$8$ have suggested that the atom and the molecule of hydrogen give out the same line spectrum and that the most deflected maximum is due to the atoms, the other to the molecules. If the atom and the molecule acquired the same kinetic energy by falling through the potential difference between the terminals of the discharge tube, the velocity of the atom would be $$\sqrt{2}$$ times that of the molecule and Gehrcke and Reichenheim found in the plates that came under their observation that the ratio of the dis- placements of the two maxima was approximately equal to $$\sqrt{2}$$. This, however, does not seem by any means always to be the case, as the following table, taken from a paper by Stark,$9$ of the results obtained by different observers, shows.

Ratio of Displacements of the Two Maxima

 * {| style="text-align: center;width: 200px;"

! ! Observer.
 * 1.75
 * Stark and Steubing
 * 1.65
 * Paschen
 * 1.58
 * 1.50
 * Stark and Steubing
 * 1.63
 * Paschen
 * 1.45
 * Strasser
 * 1.40
 * Strasser
 * 1.37
 * Stark and Steubing
 * }
 * 1.45
 * Strasser
 * 1.40
 * Strasser
 * 1.37
 * Stark and Steubing
 * }
 * Stark and Steubing
 * }

The photographs taken of the positive rays under electric and magnetic forces show also that in certain cases the velocities of the particles are grouped round certain values, for we find that some of the parabolas have a very decided beaded appearance: each bead corresponds to a group of particles moving with pretty nearly the same velocity. An example of this is shown in Fig. 17, Plate I. The intensity curve corresponding to the Doppler effect ought to have the same type of variations in intensity as these parabolas, and a beaded parabola ought to give rise to a Doppler curve with as many maxima as there are beads on the parabola. Sometimes these beads on the parabolas are quite numerous.

It is remarkable that the parabola corresponding to the atom of hydrogen is often beaded in such a way that the velocity of the particles producing one bead is to that producing the other as $$\sqrt{2}:1$$. Thus to explain the maxima in the Doppler curve with displacements in this proportion it is not necessary to assume that the molecules give out the same spectrum as the atom. The occurrence of singly charged atoms of hydrogen with velocities in this proportion of $$\sqrt{2}$$ to 1 might be accounted for in some such way as the following: the atoms with the larger velocity have been charged atoms during the whole of their career; they were atoms before they passed through the cathode and continue in this state after emerging from it; the atoms with the smaller velocity were part of a charged molecule before passing through the cathode; the molecule would only acquire a velocity$$1/\sqrt{2}$$ that of the atom. After passing through the cathode and before being deflected by the electric and magnetic fields this charged molecule breaks op into two atoms, while a the other is uncharged.

The Döppler effect we have been considering is by the "series spectrum" of hydrogen. In addition  to this spectrum hydrogen gives a second spectrum containing a great number of lines, and this spectrum Is developed,, not so brightly as the series spectrum, when positive rays pass through hydrogen. Stark$10$ has shown, and his results have been confirmed by Wilsar,$11$ that the lines in the second spectrum of hydrogen do not show the Döppler effect with the positive rays. We infer from this that the second spectrum of hydrogen is not due to any of the constituents of the positive rays. This result illustrates the importance of this method for questions relating to the origin of spectra.

Another illustration of this is the case of oxygen. Oxygen gives a series spectrum, a spark spectrum which has not been resolved into series, and some banded spectra. All these spectra are emitted when oxygen positive rays pass through oxygen, the spark spectrum being the brightest With oxygen it is the spark lines that show the Döppler effect. Wilsar and Paschen could not detect any such effect with the series lines. Stark, however, who used very large dispersions, found the effect in some of the lines; the intensity of the displaced lines was, however, very small compared with that of the undisplaced lines, while in the spark lines the displaced intensity$12$ is quite comparable with the normal intensity.

Nitrogen has a line spectrum which has not been resolved into series, and some banded spectra. The line spectrum and one of the banded spectra are found where nitrogen positive rays go through nitrogen; the banded spectrum does not show the

Döppler effect. Some of the lines in the line spectrum show it very distinctly, while it is quite absent from others (Herman, Wilsar).$13$ A very interesting point about the effect in nitrogen is that even for those lines which show the effect the value of Δλ/λ is not constant. Wilsar gives the following table for the Döppler effect for some of the nitrogen lines:—

!Wave Length. !Δλ/λ Thus the effect for the line 3995.2 is much less than for any of the others showing that the velocity of the source of this line is considerably less than that of the sources of the others. The different states in which nitrogen occurs in the positive rays are atoms with two charges, atoms with one charge, molecules with one charge, and in exceptional cases atoms with three charges and a tri-atomic molecule with one charge. If the majority of the lines were given out by the doubly charged atom and the line 3995.2 by the singly charged one we should get relative values of Δλ/λ, approximately equal to those in the preceding table.
 * 5002.9
 * 11.4
 * 4643.4
 * 10.35
 * 4630.9
 * 10.14
 * 4530.3
 * 10.14
 * 3995.2
 * 6.90
 * }
 * 10.14
 * 3995.2
 * 6.90
 * }
 * }

Stark's experiments have shown that the source of the series lines is one of the constituents of the positive rays: the question is, which constituent. We have seen that in hydrogen, for example, we have positively and negatively charged atoms, as well as neutral ones: we have also positively charged and neutral molecules. There is considerable difference of opinion as to which of these is responsible for the series lines in the hydrogen spectrum. All theories concur in regarding the atom and not the molecule as the source of these lines, but according to Wien's theory the in the neutral state, while Stark maintains that the radiation Is emitted when the atom has a positive charge: according to his the lines emitted by the neutral atom are far away in the ultraviolet.

The pressures at which spectroscopic observations have been made are so high that an atom is continually passing backwards and forwards between the neutral and charged conditions. It is thus a matter of great difficulty to determine whether the atom emits the lines in one state or the other, and there Is, I think, at present no experiment which is absolutely decisive between the two views. Thus, for example, it is found that the Doppler effect is increased when the positive rays are exposed to an accelerating potential after passing through the cathode. This, however, does not prove that the particles are charged when giving out the light, for the particles which are uncharged at one time have at other times a positive charge and so would be accelerated.

Perhaps the strongest argument in favour of the radiating particles being positively charged is that in certain cases, as Reichenheim has shown, the anode rays (see p. 84) show the Döppler effect, but even this is not conclusive, as some of the positively charged particles might have been neutralized after they had acquired their high velocity under the electric field.

There is another view as to the origin of the radiation which explains in a simple way some of the characteristic properties of the Döppler effect: this is that the light, is given out by particles which have just been neutralized by union with a negatively electriied corpuscle. The corpuscle falls into the positively charged atom and the energy gained by the fall is radiated away as light. On this view the intensity of the light should vary with the number of recombinations of positive ions and negative corpuscles. Let n be at any instant the number of neutral particles per unit volume moving velocity V, p the number of positive particles moving with the same velocity, N the number of corpuscles In the tract of these particles per unit volume.

Then the number of recombinations per second will be

 * $$pNf(v) \,$$

when f(v) is a function of v which will vanish when v Is very large, for recombination will not take place If the relative velocity of the positive particle and the corpuscle exceeds a certain value.

The number of neutral particles Ionized per second will be

 * $$nNF(v) \,$$

where F(v) Is a function of v which vanishes when v Is very small, for If the particle is to be ionized by a collision the relative velocity of the particle and corpuscle must exceed a critical value.

When the composition of the beam of positive rays has become steady the number of ionizations must equal the number of recombinations, hence

 * $$pNf(v) = nNF(v) \,$$

and therefore

 * $$ = \frac {(p+n) N f(v)F(v) }{f(v) + F(v)} \,$$

Since f(v)=0 when v = infinity and F(v) = 0 when v=0, f(v)F(v) will have a maximum for a certain value of v which will not however depend on the potential difference between the electrodes In the discharge tube. The factor p+n, the total number of positive rays charged or neutral whose velocity is v will also be a function of v and this function will depend upon the value of E, the potential difference between the electrodes In the discharge tube, for evidently if E increases, the value of v for which p+n is a maximum will increase too. On the view we are considering, the intensity of the light showing a Döppler effect corresponding to the value v will beproportional to the number of recombinations of positive ions moving with this velocity with negatively electrified corpuscles. It will thus be proportional to pNf(v) which we have seen is equal to

 * $$ (p+n) \Bigg\{ \frac {f(v)F(v) }{f(v) + F(v)} \Bigg\} N\,$$

The second factor in this expression

 * $$ \frac {f(v)F(v) }{f(v) + F(v)} $$

has its maximum value for a value of v which does not depend upon the potential difference: the other factor (p + n) does depend upon this potential difference. Thus the value of v for which the product of these factors is a maximum will depend to some extent on E, but since the value of v which makes one of the factors a maximum is quite independent of E we should expect that the variation with E of the velocity which makes the product a maximum would be smaller than the variation in the average velocity of the particles in the positive rays.

Again since F{v) vanishes when v is less than a certain value v$0$ there will be no light showing a Döppler effect corresponding to a velocity less than v$0$, thus there will be a dark space between the original line and the displaced lines. This also is in accordance with the observations. Since f{v) vanishes when v is greater than a certain value v, there will be no Doppler effect showing a greater displacement than that corresponding to v. Though it has not perhaps been absolutely proved there are strong indications that the Doppler effect cannot be increased beyond a certain definite value, however large the potential applied to the discharge tube may be.

The spectroscopy of the positive rays suggests some very interesting questions, as, for example, what kind of light do the molecules emit? In the positive rays in hydrogen the molecules frequently outnumber the atoms, but no radiation that can be attributed to the molecules has yet been detected.The spectrum of hydrogen is there, but as it does not show the Döppler effect it cannot be due to the molecules. It would seem as if the molecules must either give rise to a continuous spectrum or else to one in the infra red.

As the second spectrum of hydrogen is present when the positive rays pass through a gas, but does not show any displacement, it must arise from some process in which the moving particles do not take part, such for example as the combination of a positive atom with a negative one (not with a corpuscle) to form a neutral molecule.

Stark$14$ has detected by the increased Döppler effect lines due to the doubly and triply charged atoms of mercury and to the doubly charged atom of helium. He finds that the lines given out by the multiply charged atoms belong to different series in Faschen and Runge's classification from those given out by atoms with only one charge.

When positive rays produced in a gas A pass through a gas B the spectra of both A and B are given out: Wilsar, "Phys. Zeitschr.," 12, p. 1091, and Fulcher (ibid. 13, p. 224), have shown that all the lines of A are displaced while all those of B are in their normal position. A bibliography of the Döppler effect in the Positive Rays has recently been published by Fulcher, " Jahrb. d. Radioaktivitat," X, p. 82, 1913.

$1$ Stark, "Physik. Zeitschr.," 6, p. 892, 1905. "Ann. d. Phys.," XXI, p. 451, 1906.

$2$ "Ann. der. Phys.," 28, p. 974.

$3$ "Ann. der Phys.," 27, p. 599.

$4$ Ibid., 28, p. 974.

$5$ "Ann. der Phys.," 23, p. 247, 1907,

$6$ Ibid., 28, 978.

$7$ Ibid., 31, 890, 1910

$8$" Verh. d. Deutsch. Phys. Ges.," 12, p. 414, 1910.

$9$ Ibid., 12, p. 711,1910.?

$10$ Stark, " Ann. der Phys.," 21, p. 425, 1906.

$11$ "Ann. der Phys.," 39, p. 1251,1912.

$12$ Paschen, "Ann. der Phys.," 23, p. 261, 1907.

$13$ "Phys. Zeit.," 7, p. 568,1906.

$14$ "Ann, der Phys.," XL, p. 499, 1913 ; XLII, p. 241, 1913.


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Rays of Positive Electricity and Their Application to Chemical Analyses/Spectra Produced by Bombardment with Positive Rays

The spectra produced when the positive rays strike salts of the alkali metals are very interesting. The salts give out the lines of the alkali; for example LiCI give out the red lithium line and sodium salts the D line. It is remarkable the lines due to the metal are more easily excited in the salts the than in the metal themselves. Thus if the liquid alloy of sodium and potassium is bombarded by positive rays the specks of oxide on the surface glow brightly with the sodium light while the clean surface remains quite dark. Some observers have noticed what seems a similar effect with hydrogen, viz. that the hydrogen lines are more easily excited in water vapour than in pure hydrogen. The fact that in the positive ray photographs, the parabolas corresponding to a certain type of ray, for example the carbon or oxygen atom with two charges, is more easily developed from compounds than from the molecules of the gases themselves is probably connected with this effect

The production of spectra by bombardment with cathode rays has been investigated by Gyllensköld ("Ark. f. Math. Ast. oet. Fys.," 4, No. 33, 1908), and by Stark and Wendt ("Ann. der Phys.," 88, p. 669, 1912) who have shown that the colourless salts of the alkalies and alkaline earths and also of thallium, zinc, and aluminium give out the series lines of the metal when struck by the positive rays and that the lines given out do not depend upon the character of the salts. According to Stark and Wendt the seat of the emission is not the surface of the salt itself but a layer of gas, less than 1 mm. thick, close to the surface. This layer is analogous to the velvety glow which covers the surface of the cathode where an electric discharge passes through a gas at a low pressure.

To develop the spectrum of the metal the positive rays must have more than a certain critical amount of energy depending on the nature of the salt. The values of V, this critical energy, measured by the number of volts through which the atomic charge must fall to acquire it, have been recombinations measured by Stark and Wendt and are given in the following table :—
 * {| style="text-align: center;width: 500px;"

! Metal ! Salt ! Light given out ! V
 * Lithium
 * chloride
 * red
 * 600
 * oxide
 * red
 * 600
 * λ 6710
 * <800
 * Sodium
 * chloride
 * yellow light
 * 750
 * Potassium
 * chloride and oxide
 * λ 580
 * <2400
 * Rubidium
 * sub-oxide
 * λ 572
 * <3500
 * Caesium
 * chloride
 * λ 566
 * <4500
 * Magnesium
 * chloride
 * λ 518
 * <1200
 * Calcium
 * fluoride
 * red violet light
 * 150
 * carbonate
 * 1500
 * sulphate
 * 1500
 * oxide
 * 1400
 * Strontium
 * chloride
 * λ 496
 * <2500
 * Barium
 * λ 554-493
 * <2500
 * Thallium
 * sulphate
 * λ 535
 * 4500
 * Aluminium
 * oxide
 * λ 396
 * <4500
 * Zinc
 * λ 475
 * <4600
 * }
 * 1400
 * Strontium
 * chloride
 * λ 496
 * <2500
 * Barium
 * λ 554-493
 * <2500
 * Thallium
 * sulphate
 * λ 535
 * 4500
 * Aluminium
 * oxide
 * λ 396
 * <4500
 * Zinc
 * λ 475
 * <4600
 * }
 * λ 396
 * <4500
 * Zinc
 * λ 475
 * <4600
 * }
 * <4600
 * }

It must not be supposed that the amounts of energy given in the last column represent the minimum amount required to excite the particular kind of light given in the third column. When energy has to be transferred from a charged atom to a corpuscle, the latter only receives a very small fraction of the energy of the atom, thus a very small fraction of the energy of the positive rays may be transformed into a kind available for light production.

Gyllensköld observed that in addition to the D lines sodium chloride gives out a series of bands in the blue and Stark and Wendt have shown that for this to occur the energy of the rays must exceed a critical value which in most cases is less than that required to excite the line spectrum.


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Rays of Positive Electricity and Their Application to Chemical Analyses/Disintegration of Metals Under the Action of Positive Rays

When positive rays strike against a metallic surface, the metal disintegrates and forms a deposit on the walls of the tube surrounding the metal. A well-known example of this is the "spluttering" of the cathode in a vacuum tube; another is observed when working with an apparatus like that in Effect at Very Low Pressures|Fig. 10; after long use the thin metal tube which through the cathode gets worn away at the end nearest the discharge tube, as if it had been struck by a sand blast. Sometimes several millimetres of the tube are destroyed in this way. An excellent account of the very numerous experiments which have been made on the spluttering of the cathode will be found in a report by Kohlschiitter ("Jahrbuch der Radioaktivitat," July, 1912).

The experiments of Holborn and Austin, Granqulsts and Kohlschütter indicate that with a constant current w the loss of weight in a given time may be represented by a formula of the type

 * $$w=a\frac{A}{n}(V-S)$$

where V is the cathode fall of potential, A the atomic weight of the metal, n a small positive integer, and a and S quantities which are much the same for all metals, or at any rate the metals can be divided into large classes and a, and S are the same for all the metals in one class. For a current of .6 milliamperes, Holborn and Austin found that for all the metals they tried S was 495 volts. We see that a formula, of this type implies that there is no spluttering unless the cathode fall of potential exceeds a definite value S and this seems to be verified by experience.

The experiments of Holborn and Austin, Kohlschiitter and others have shown that this expression for the loss of weight of the cathode fails when V exceeds a certain value, for hydrogen this value seems to be so low that the expression fails before the loss of weight becomes measurable.

The loss of weight of the six metals Al, Fe, Cu, Pt, Ag, Au have been measured by Kohlschütter and Müller ("Zeitschr. f. Elektroch.," 12, 365, 1906) and Kohlschiitter and Goldschmidt (ibid. 14, 221, 1908) in the gases H$2$, He, Na, O$2$ and Au, under as nearly as possible identical electrical conditions. They found that for all gases the amount of weight lost was in the order in which the metals are written above, gold always losing the greatest amount and aluminium the least. For the same meta! in different gases the loss of weight followed the order of the atomic weight of the gases, the loss in hydrogen being least and that in argon greatest. This may be connected with the fact that (see p. 47) elements of high atomic weight acquire multiple charges of electricity more easily than the lighter elements, and atoms with a multiple charge have more energy when they strike against the cathode than those which have only one charge. It is easy to understand in a general way why particles with the large amount of energy possessed by the positive rays when they strike against an atom in the cathode might communicate to it sufficient energy to enable it to escape from the cathode. A complete theory, however, is lacking, and one which would explain some of the more striking facts, such as why the value of S is so nearly the same for metals of very different physical and chemical properties, would probably throw a good deal of light upon some important properties of the atom.

Dechend and Hammer ("Zeitschr. f. Elektroch.," 17, 235) allowed the positive rays produced in sulphuretted hydrogen to pass through a perforated cathode and after deflection by magnetic and electric fields to fall upon a plate of polished silver, they could detect the parabolas on the plate, but while the parabolas due to hydrogen were so faint that they could only be detected as breath figures, those due to the heavier atoms, presumably sulphur, had so affected the plate that they could not be removed either by acid or rubbing. The greatest effect, however, was produced by the undeviated rays. In addition to the effects produced when the positive rays strike against a metal plate there is, as Schmidt has shown, a general oxidation over the surface when the metal is oxldizable and when the gas surrounding it contains oxygen. The passage of the positive rays through the oxygen produces atomic oxygen which is very active chemically and which attacks the plate. If, on the other hand, an oxidized plate is placed in hydrogen and exposed to the action of positive rays the oxide is reduced, the rays produce atomic hydrogen which acts as a strong reducing agent.

Some of the atoms constituting the positive rays seem to enter a metal against which they strike, and either combine with the metal or get absorbed by it. Helium, neon, and mercury vapour seem especially noticeable in this respect. If a cathode has once been used for any of these gases, positive rays corresponding to these elements will be found when the cathode is used with other gases, and it requires long continued discharge and repeated fillings with other gases before they are eliminated.

A very valuable Bibliography of Researches on Positive Rays has been published by Fulcher (Smithsonian Miscellaneous Collection, 5. p. 295, 1909).


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Page:Best Russian Short Stories.djvu/185

Rh of Lelechka. Lelechka was in her thoughts, first a sweet, tiny girl, then a sweet, big girl, then again a delightful little girl; and so until the end she remained mamma's little Lelechka.

Serafima Aleksandrovna did not even notice that Fedosya came up to her and paused before her. Fedosya had a worried, frightened look.

"Madam, madam," she said quietly, in a trembling voice.

Serafima Aleksandrovna gave a start. Fedosya's face made her anxious.

"What is it, Fedosya?" she asked with great concern. "Is there anything wrong with Lelechka?"

"No, madam," said Fedosya, as she gesticulated with her hands to reassure her mistress and to make her sit down. "Lelechka is asleep, may God be with her! Only I'd like to say something—you see—Lelechka is always hiding herself—that's not good."

Fedosya looked at her mistress with fixed eyes, which had grown round from fright.

"Why not good?" asked Serafima Aleksandrovna, with vexation, succumbing involuntarily to vague fears.

"I can't tell you how bad it is," said Fedosya, and her face expressed the most decided confidence.

"Please speak in a sensible way," observed Serafima Aleksandrovna dryly. "I understand nothing of what you are saying."

"You see, madam, it's a kind of omen," explained Fedosya abruptly, in a shamefaced way.

"Nonsense!" said Serafima Aleksandrovna.

She did not wish to hear any further as to the sort of omen it was, and what it foreboded. But, somehow, a sense of fear and of sadness crept into her mood, and it was humiliating to feel that an absurd tale should disturb her beloved fancies, and should agitate her so deeply.

"Of course I know that gentlefolk don't believe in omens, but it's a bad omen, madam," Fedosya went on in a doleful voice, "the young lady will hide, and hide. . . ."

Suddenly she burst into tears, sobbing out loudly: "She'll hide, and hide, and hide away, angelic little soul, in a damp


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Rays of Positive Electricity and Their Application to Chemical Analyses/On The Use of the Positive Rays for Chemical Analysis

1. We shall now proceed to show how the method of positive rays supplies us with a very powerful method of chemical analysis, and how from the study of the positive ray photographs we are able to determine the different kinds of atoms and molecules in the discharge tube. Each atom or molecule in the discharge tube produces a separate parabola on the photographic plate and if we measure these parabolas then by means of the formula

 * $$\frac{e}{m}=\frac{y^2A}{xB^2}$$

we can determine the value of e/m for the particles producing any of the parabolas which occur on the photographic plate. We know, too, that the charge e is either the ionic charge, whose value on the electrostatic system of units is 4.8 x 10$3$, or some multiple of it. We have, too, as we shall see, the means of determining what this multiple is. As we can determine the value of e and since we know by the measurement of the parabola the value of e/m we can deduce the value of m and thus determine the masses of the particles forming the positive rays. As these particles are the atoms and molecules of the gases in the discharge tube it is evident that in this way we can determine the atomic or molecular weight of the gases in the positive rays. We can thus identify these gases as far as can be done by the knowledge of their atomic weight. The study of the photographs gives us in fact the atomic weights of the various gases in the tube and thus enables us to determine the nature of the contents of the tube. We can thus analyse a gas by putting a small quantity of it into a discharge tube and taking a photograph of the positive rays. This method of analysis has many advantages. In the first place only a very small quantity of gas Is required, the total amount of gas In the discharge tube of the size described on page 16 would only occupy about .01 c.c. at atmospheric pressure and a constituent present to the extent of only a very small percentage would give well defined parabolas. If there Is a new gas In the tube it is indicated by the presence of a new parabola, but this parabola does far more than show that something new is present, it tells us what is the atomic weight of the new constituent. Let us compare for a moment the method with that of spectrum analysis. We might detect a new gas by observing an unknown line in the spectrum when the electric discharge passed through the gas. This observation would, however, tell us nothing about the nature of the substance giving the new line, nor, indeed, whether it arose from a new substance at all: it might be a line given out by a well-known substance under new electrical conditions. Again, If a substance is only present to the extent of a few per cent It very often happens that its spectrum is completely swamped by that of the more abundant substance: thus, for example, in a mixture of helium and hydrogen we cannot observe the helium lines unless the helium is a considerable percentage of the mixture.

This is not the case with the positive rays, or at any rate not to anything like the same extent; the presence of one per cent of helium would be very easily detected by the positive rays. The method, too, is more sensitive than that of spectrum analysis. With the apparatus described above the helium in 1 c.c. of air, i.e. about 3 x10$-10$ c.c., could be detected with great ease when it formed only about one per cent of the mixed gas in the tube. No special attention was paid to making this particular apparatus specially sensitive. To get the best results the size of the tube running through the cathode has to be chosen with reference to the distance of the photographic plate from the cathode, and other circumstances; this was not done in the apparatus under discussion, nor were the photographic plates used of any special sensitiveness; by attention to these points the sensitiveness of the method could very materially be increased.

Again the method of the positive rays enables us when we have found the substance to say whether the molecule is monatomic or diatomic; if it is diatomic we shall have two new parabolas, one indicating particles with a mass twice that of the picartles producing the other; if the molecule is monatomic there will be only one parabola unless the particle acquire a double charge and the presence of this extra charge can be recognized by the tests previously described. The method of the positive rays has the advantage of revealing the presence of the molecules of compound gas as well as the atoms and molecules of elementary substances. Since different compounds may have the same molecular weight there is sometimes ambiguity in interpreting the photographs produced by the positive rays; for example, CO$6$ and N$2$O produce the same parabolas as also do CO and N$2$ In such cases to find out the origin of such a parabola we must repeat the experiment under different conditions; for example, if we put something in the tube which absorbs CO$2$ and not N$2$O and find that the parabola disappears, we conclude that it was due to CO$2$; If it does not disappear it is not due to CO$2$, but to N$2$O or some other compound with the same molecular weight.

2. The ambiguity as to whether a line with a value of m/e equal say to 8 (m/e for the hydrogen atom being taken as unity) is to be ascribed to an atom with atomic weight 8 carrying a single charge or to one with an atomic weight 16 carrying two charges or to one with atomic weight 24 with three charges may be removed by the considerations given on page 47. For example, If the particle producing this parabola A carries a double charge there will be another and more intense parabola B for which the value of e/m is twice that for A, and the parabola B will have a prolongation towards the vertical axis, the distance of the head of this prolongation from the vertical axis being half the distance of the heads of the normal para- bolas (see p. 47). If A represents a particle with a threefold charge there will be another stronger parabola B for which e/m has three times the value corresponding to the parabola A, and B will have a prolongation towards the vertical axis extending to one third of the normal distance.

3. For the purposes of Chemical Analysis It is not necessary to use the elaborate apparatus shown in Fig, 10, the simpler one shown in Fig. 6 is all that Is required for this purpose. The more elaborate apparatus is only required when we require to know accurately the values of the quantities A and B which occur In the expression for e/m.

For the determination of the masses of the particles producing the different parabolas the measurement of the quantities A and B Is unnecessary if we can recognize the particle which produces any particular parabola. For since A and B are the same for all the parabolas, then for any two parabolas we have by the equation on page 106

 * $$\frac{(e/m)_1}{(e/m)_2} = \frac{y_1^2/x_1}{y_2^2/x_2}$$

when e/m$2$ and e/m$1$ are the values of e/m, for the particles producing the parabolas (1) and (2) respectively, (x$2$y$1$) (x$1$y$2$) are the co-ordinates of any point on the first and second parabolas respectively.

If the points on the two parabolas have the same values of x so that x$2$= x$1$

then

 * $$\frac{(e/m)_1}{(e/m)_2} = \frac{y_1^2}{y_2^2}$$

if the charges are the same

 * $$\frac{m_2}{m_1} = \frac{y_1^2}{y_2^2}$$

As the line corresponding to the atom of hydrogen occurs on al! the plates and can at once be recognized by being the most deflected line on the plate, the value of (e/m} for the particles producing any parabola can be at once, by the aid of this formula, compared with the value of this quantity for an atom of hydrogen and the masses of the various particles thereby determined,

A convenient instrument for making the necessary measurements is shown in Fig. 14. The plate is inserted in the holder A. The camera is arranged so that the direction in which the rays are deflected by the magnetic force alone (the vertical axis in the preceding figures) is parallel to the longer side of the photographic plate. The deflection due to the electrostatic field is at right angles to this and parallel to the shorter side of the plate. The plate is placed in the holder so that the axis of no electrostatic deflection is parallel to B, and that of no magnetic deflection perpendicular to BB. A needle NN whose point comes close to the plate is placed in the carrier C which can move parallel to BB by sliding along BB, and perpendicular to it by means of the screw S, the position of the carrier is read by two verniers V$2$ and V$1$. There is always a circular patch of some size on the plate at the place, where the undeflected particles hit the plate: the zero is at the centre of the spot. By putting the needle first at the centre of the spot, then moving the carrier through a certain distance perpendicular to BB by the screw S, and sliding the carrier parallel to BB until the needle comes on the parabolas In turn, the values of y for the different parabolas corresponding to a constant value of x can be measured.

The equation page 110 then enables us to find the ratio of the  of masses of the particles producing the different parabolas. We can avoid any uncertainty as to the position of the zero by taking two photographs, the electrostatic field remaining the same in the two, while the magnetic field in the first photograph is equal in magnitude but opposite in direction to that in the second. Thus each kind of particle will now give two parabolic arcs, as in Fig. 38, and the distance between two points AB situated on the same vertical line will be twice the vertical deflection due to either magnetic field. As these arcs are much finer than the central spot, the distant AB can be measured with greater accuracy than either deflection separately.

The advantages of the method are illustrated by the photo- graphs reproduced in Figs. 47 and 48, Plate IV. These re- ' present the parabolas obtained when the discharge passes through the residues of liquid air; Fig. 47 represents the curve for the residues which had been treated so as to include the heavier constituents of the atmosphere; Figs. 48, Plate IV. and 49, Plate V. when the treatment had been such as to retain the lighter constituents.

When the plate for the heavier gases is measured up, it shows a faint line corresponding to the atomic weight 128 (xenon), a very strong line corresponding to an atomic weight 82 (krypton), a strong argon line 40, and the neon line 20. There are no lines on the plate which cannot be ascribed to known elements, and hence we may conclude that In the atmosphere there are no unknown gases of large atomic weight occurring In quantities comparable with those of xenon and krypton. This is a good example of the convenience of this method of analysis, for a single photograph reveals at a glance all the gases In the sample analysed. This photograph shows very plainly the existence of multiply charged atoms of the monatomic gases. The neon parabola extends towards the vertical to within half the normal distance of the heads of the parabolas; this shows that some of the neon atoms carry a double charge, and this is confirmed by the presence of a line on the plate for which e/m has twice the value corresponding to the neon line. The argon parabola approaches the vertical even more closely than that representing neon, as it begins at a distance from the vertical only one-third the normal distance, showing the argon atom can have as many as three charges. The krypton line approaches to within one-quarter of the normal distance showing that the krypton atom may have as many as four charges. We see from this how the maximum charge acquired by atoms of elements belonging to the same group increases with the atomic weight of the element

Let us now consider the photograph taken with the lighter constituents (Fig. 48, Plate IV.): here we find the line corresponding to helium; to neon, this is very strong and there is also a line corresponding to the neon atom with a double charge; to argon, and in addition there is a line corresponding to an element with an atomic weight 22. A molecule of carbonic acid with a double charge would give a line in this position, but this cannot be the origin of the line as the carbonic acid can be removed from the gas without producing any change in the brightness of the line. This line is much fainter than the neon line so that in the atmosphere the quantity of the gas which is the source of this line is small compared with that of neon.

The origin of this line presents many points of interest: there is no recognized element with this atomic weight, nor are there any compounds of recognized elements which would satisfy these conditions. It must, I think, be a new element.

For though the compound NeH$2$ would have the required mass, there is strong evidence that the Is due to an element. Thus we find on the plate another line for which m/e =11, and the line for which m/e = 22 has a prolongation half way to the axls, showing that the particle exists with a double charge; this Is a frequent occurrence with an atom of an element but I do not know of any case of a molecule of a compound possessing more than one charge.

If we accept Mendeleefs table there is no room for an element with such an atomic weight as 22 unless we suppose that near neon we have a group of two or more elements with similar properties, just as in another part of the table there Is the group iron, nickel, and cobalt

Mr. F. W. Aston has made at the Cavendish Laboratory many attempts to separate this new gas from neon whose atomic weight Is 20. The method he first tried was to fractionate a mixture of the two gases by means of their absorption by cocoanut charcoal cooled by liquid air. This absorption in the case of most gases Increases with the atomic weight, and though the difference between 20 and 22 — the atomic weights of the two gases In the mixture — is but small, he devised an apparatus by means of which the absorption was repeated so frequently that If there had been as great a difference In the absorption as from the analogy with other gases we might have expected from the difference in their atomic weights, the proportion between neon and the new gas would have been appreciably altered by the treatment. He could, however, find no difference whatever In this proportion before and after fractlonation. To measure this proportion he used two methods, (1) by comparing the intensity of the two lines In the positive ray photograph and (2) by measuring the density of the mixture by means of a specially constructed quartz balance which would have detected an alteration of very few per cent in the proportion between the two.gases. We conclude, therefore, that the physical properties of the two gases are much more nearly equal than we should have expected from their atomic weights.

Another method of fractionation. used by Mr. Aston was more successful, this was to allow the mixed gases to diffuse through a porous substance such as the stem of a clay tobacco pipe. The lighter constituent diffuses faster than the heavier one and by this means he obtained sufficient alteration in the proportion between the two gases to produce appreciable changes in the relative brightness of the two lines on the positive ray photograph, and changes in the density large enough to be detected by the quartz balance. No difference, however, could be observed in the spectrum of the mixture, and this in conjunction with the failure of the cooled charcoal to produce any separation gives some grounds for the suspicion that the two gases, although of different atomic weights, may be indistinguishable in their chemical and spectroscopic  properties. There are several products of radio-active transformations such as radio-lead and thorium which have different atomic weights and are supposed to be inseparable from each other by any chemical process.

As another example of the method we will take its application to the investigation of the gases given off when solids are bombarded by cathode rays. The apparatus used for this is shown in Fig 13. B is the vessel in which the positive rays are produced. A is a vessel communicating with B by two tubes, one of which is a very fine capillary tube while the other is 5 or 6 mm. in diameter; taps are inserted so that one or both of these tubes may be closed and the vessels isolated from each other. The vessel A contains a curved cathode such as are used for Röntgen ray focus tubes, and the cathode rays focus on the platform on which the substance to be bombarded is placed. After the metal or other solid under examination has been placed on the platform, the taps between A and B are turned and A is exhausted by a Gaede pump until the vacuum is low enough to give cathode rays. The electric discharge is then sent through A and the solid on the platform bombarded. The result of the bombardment is that in a few seconds so much gas, mainly CO$2$ and hydrogen, is driven out of the solid that the pressure gets too high for the cathode rays to be formed, and unless some precautions to lower the pressure were taken the bombardment would stop. To avoid this a tube containing charcoal cooled by liquid air is connected with A, the cooled charcoal absorbs the CO$2$, and enough of the hydrogen to keep the vacuum in A low enough to give cathode rays.

To see what gases are given off in consequence of the bombardment, a photograph of the positive rays is taken when the connexion between A and B is cut off. After this is finished,, and when the bombardment has gone on for several hours, the taps between A and B are turned and the gas from A is allowed to go into B; another photograph is taken. The lines in the second photograph which are not in the first represent the gases which have been liberated from the solid by the bombardment with cathode rays. Fig. 50, Plate V., represents two such photographs; (a) that taken before turning the tap and (b) after. In (b) there are the following lines which do not occur in (a): (1) a very strong line corresponding to a substance with atomic weight 3; (2) one corresponding to helium, atomic weight 4, generally much fainter than the 3 line, and (3) lines representing neon with one and two charges. The amount of helium and neon are so small that their spectral lines were not visible when the discharge through the tube was examined with the spectroscope. Nearly every substance I have tried, including platinum, palladium, aluminum, copper, zinc, Iron, nickel, silver, gold, lead, graphite, diamond dust, lithium chloride, two specimens of meteorites and a large number of metallic salts, give out, when first they are bombarded, helium as well as the substance giving the 3 line; the amount of helium given off generally falls off very considerably after the bombardment has been prolonged some hours; the substance giving the 3 line is, however, much more persistent, and In some cases, for example that of KHO, the bombardment may be prolonged for several weeks without any diminution In the rate of evolution of the gas.

The presence of mercury vapour In the vessel A diminishes very much the intensity of the 3 line; hence we may conclude, I think, that the substance which gives the 3 line combines with mercury vapour when an electric discharge passes through a mixture of the two gases. Another case where the presence of one gas causes the disappearance of the lines due to another is that of oxygen and mercury vapour; the mercury lines are not seen in the photographs of the positive rays when the gas in the tube Is mainly oxygen, although with most gases these are about the strongest lines on the photographic plate. The disappearance of the mercury lines In this case may easily be explained by the combination of the mercury vapour with the oxygen.


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The Encyclopedia Americana (1920)/Inner Light, The

INNER LIGHT, The, or INWARD LIGHT. A doctrine especially emphasized by the Quakers. The teaching is that within every soul there is a revelation of God. There is a revelation of God in Nature, there is a revelation of God in the Bible, but there is also the Inner Light revealing God. It may be only a seed and may not come to perfection, for its call may be denied. It differs from the Methodist doctrine of the witness of the Spirit in that it is recognized as always residing in the heart, while the witness of the Spirit comes from without through experience. It differs from the conscience, because its function is not to determine the difference between right and wrong but to illume the soul and make it aware of the presence of God. Consult Barclay, Robert, ‘An Apology for the true Christian Divinity’; Bradford, Amory H., ‘The Inward Light’ (1905).


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Rays of Positive Electricity and Their Application to Chemical Analyses/On the Nature of X3 the Substance Giving the 3 Line

The only known substances which could give the line with the value of mfe three times that of the hydrogen atom are :

(1) an atom of carbon charged with four units of electricity, and

(2) a molecule containing three atoms of hydrogen.

The first of these alternatives must be abandoned  for the following reasons: (1) we have seen that a line corresponding  to  multiple charge on an atom is accompanied, unless  the pressure is exceedingly low, by certain peculiarities in the line corresponding to the atom with one charge; for example, If were a line corresponding to a carbon atom with two charges, the line corresponding to the carbon atom with one would be prolonged until its extremity was only one-half the normal distance from the vertical axis; If there were a line corresponding to the carbon atom with three charges, the ordinary carbon line would be prolonged until its distance from the vertical was only one-third of the normal distance, while a carbon atom with four charges would prolong the ordinary carbon line to within one-quarter of the distance from the axis. Again the greater the charge the less the intensity, so that a line due to a quadruply charged carbon atom would be accompanied by a stronger line due to a triply charged atom, a still stronger one due to a doubly charged atom, while the normal carbon line would be the strongest of all.

Now in the case of the 3 line we do not find any of these characteristics; the carbon line is not prolonged to within one-quarter of the normal distance, and so far from the line being accompanied by a stronger line due to a doubly charged carbon atom, in many of the cases where the 3 line is strongest the line due to the doubly charged atom is not strong enough to be detected; indeed in some of these cases the 3 line is stronger than the normal carbon line.

Again since the gas giving the 3 line can be stored In the vessel A for days after the bombardment has ceased, if this line were due to carbon with four charges it must be because some carbon compound Is formed by the bombardment, which when introduced Into the discharge tube gives, when the discharge passes through it, carbon atoms with four charges. Now experiments have been made with a great variety of carbon carbons introduced directly Into the discharge tube, CH$3$, CO$4$, CO, C$2$H$2$ QH3, COCl$4$, CCI$3$, and many others, and none of these have given this line: we must therefore abandon this solution of the problem.

I find too that whenever large amounts of X$4$ are produced spectroscopic examination shows that considerable quantities of hydrogen are liberated by the bombardment; in fact the brightness in the spectroscope of the hydrogen lines in the bombardment vessel may be taken as giving a rough indication of the brightness to be expected of the X$3$ line in the positive ray photograph.

Let us next consider the connexion between the production of X$3$ and the nature of the substance bombarded by the cathode rays. We get more definite conditions for the bombarded body if we use soluble salts instead of pieces of metal or minerals. The latter may have absorbed X$3$ and contain stores of this gas in the absorbed state which are liberated when the solid is bombarded by cathode rays. If we could subject the solid before the bombardment to some process by which we could free it from absorbed gas we should expect that if the source of the X$3$ were gas absorbed by the solid, the bombardment of a substance which had been treated in this way would not give rise to any X$3$.

The most effective way of liberating the absorbed gas would seem to be to dissolve the solid in a suitable solvent and then evaporate the solution to dryness. Those salts which are soluble in water or alcohol can readily be treated in this way. I have therefore made experiments on a large number of soluble salts bombarding them before and after they have been dissolved and evaporated to dryness, and testing by the positive ray photographs the yield of X$3$ in each case. Sal-ammoniac made by allowing streams of ammonia and hydrochloric acid gas was found to give X$3$ when bombarded; in this case the possibility that X$3$ was absorbed in the salt would seem to be excluded.

I find that salts may be divided Into two classes with respect to the way in which their evolution of X$3$ is affected by solution and evaporation. One class of salts which Includes KI, Li$3$CO$2$, KCl give a very much smaller output of X$3$after this treatment than they did before; the other class which includes KOH, LiCl, LiOH, CaCl$3$ give much the same output after solution as they did before even though they are dissolved and evaporated over and over again. The salts of the first class do not contain hydrogen, while those of the second either contain hydrogen or are deliquescent and thus can absorb water from the atmosphere on their way to the bombardment chamber after evaporation. The fact that some salts continue to give supplies of X$2$ after repeated solution and evaporation shows I think that X$3$ can be manufactured from substances of definite chemical composition by bombardment with cathode rays, and the fact that such salts contain hydrogen either as part of their constitution or in water of crystallization suggests that X$3$ consists of hydrogen and is represented by the formula H$3$. The other alternative is that it is an element produced by the disintegration of some or other of the elements in the salt, but this view would not explain why its production is so closely associated with the presence of hydrogen.

One of the most convenient ways of preparing X$3$ is to bombard potash, KOH, by cathode rays. I bombarded a few grammes of potash for several months pumping off after each day's running the gases liberated by the bombardment, these consisted of H$3$. O$2$ and X$2$ and at the end of the time I could not detect any falling off in the rate of production of X$3$. By bombarding the potash, supplies of X$3$ mixed with hydrogen and oxygen were obtained and a series of experiments made with them with the object of discovering some of the properties of this gas. The method used to test for the presence of X$3$ after the mixed gases of which it was a constituent had been subjected to any treatment was to introduce a small quantity of the mixed gases into the discharge tube, take a positive ray photograph and estimate the brightness of the X$3$ line. Before such experiment the discharge tube was well washed out with oxygen and a test photograph taken to make certain that no X$3$ was In the tube before the introduction of the gas which was to be tested. One property of this gas, that of combining with mercury vapour when an electric discharge passes through a mixture of these gases, has already been mentioned. Another property was discovered accidentally: the mixed gases obtained by bombarding the potash were drawn off day by day and stored up for further tests. It was soon noticed that some of the samples kept much better than others and it seemed possible that this difference might be due to differences in the brightness of the light to which the samples had been exposed. To test this a piece of magnesium wire was burnt In front of a sample which was known to contain a considerable quantity of X$3$, with the result that the X$3$ almost disappeared. The gas exposed to the light was a mixture of hydrogen, oxygen and X$3$, if the oxygen is taken out of the mixture by absorbing it with charcoal cooled with liquid air, exposure to light produces no effect on the X$3$; the conclusion we.draw is that under the Influence of the light the X$3$ combines with oxygen. If the mixture is kept In the dark or if the oxygen Is taken out of it the X$3$ lasts for a long time, certainly for several weeks. Again If a strong spark is sent through the mixture containing oxygen so that a vigorous explosion takes place the X$3$ disappears, presumably combining with the oxygen. If the oxygen is removed the mixture of hydrogen and X$3$ will stand a good deal of sparking without any considerable diminution in the amount of X$3$.

The fact that sparking with oxygen destroys the X$3$, makes the removal of the hydrogen, which is by far the largest constituent of the mixture, a matter of considerable difificulty. The most effective way I know of increasing the proportion of X$3$ is first to remove the oxygen, then to put the mixture of H$3$ and X$2$ into a vessel to which a palladium tube is attached; when the palladium is heated to redness the hydrogen diffuses through it much more rapidly than the X$3$, though some of this gas can get through the palladium. The result is that the gas left behind in the vessel contains a much greater proportion of X$3$ than it did before. The preponderance of Hg in the original mixture is, however, so great that even by this means I have not been able to prepare any sample in which the hydrogen was not greatly in excess.

Another interesting property is that the X$3$ almost disappears when placed in a quartz tube with some copper oxide and the whole heated to a red heat.

In the absence of oxygen and copper oxide X$3$ may be heated to a high temperature without destruction. Summing up the results we see that when hydrogen is present in the substance bombarded a continuous supply of X$3$ can be obtained, while from substances which do not contain hydrogen the supply is soon exhausted. Again, under certain conditions such as exposure to bright light, vigorous sparking in the presence of oxygen, contact with glowing copper oxide, X$3$ combines with oxygen. These results seem to me to point to the conclusion that X$3$ is tri-atomic hydrogen H$3$. If this is so, its properties are very interesting. Unlike O3 its existence cannot be reconciled with the ordinary views about valency. If, however, we regard an atom of hydrogen as consisting of a positive nucleus and one negative corpuscle, it will exert forces analogous to those excited by a magnet and I can see no reason why a group of three of these arranged so that their axes form a closed ring should not form a stable arrangement.

The stability of X$3$ Is much greater than that of ozone O$3$. the latter does not persist for nearly as long as X$3$. It breaks up at a moderate temperature which would have no effect on Xj, and it disappears under a kind of sparking which would leave X$3$ undecoinposed. In fact X$3$ seems more stable than any known allotropic form of an element.

Many attempts have been made to obtain spectroscopic evidence of X$3$ by putting mixtures of this gas and hydrogen In a quartz tube and photographing the spectrum obtained when a discharge was sent through the tube, the electrodes were pieces of tin-foil placed outside the tube. No lines which could be ascribed to X$3$ were detected, the first and second spectra of hydrogen were bright, and in spite of efforts to get rid of mercury vapour the mercury lines were visible.

Bombardment by cathode rays is not the only method of obtaining X$3$. I heated by an electric current in a good vacuum a fine tantalum wire, such as are used for metallic filament lamps, until it fused and found that a considerable amount of X$3$ was given off. Some time ago I found that when the discharge from a Wehnelt cathode was sent through an exhausted tube X$3$ was liberated. I have found subsequently that It is not necessary to send the discharge through the tube, the heating of the cathode is sufficient to liberate this gas. Again if a considerable quantity of potash is placed in a vacuum and left for some time an appreciable quantity of X$3$ is liberated.


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The Encyclopedia Americana (1920)/Inquiline

INQUILINE, ĭn'kwĭ-lĭn, a term applied in zoology to animals which live as tenants within the nests or homes of other animals. The use of the term is almost entirely confined to entomology and then often restricted to the cases in which the rightful and the intruding tenants are closely related. Similar cases among other animals are commonly designated as commensalism (q.v.), but these and similar terms are used rather loosely. Examples of the inquiline relation occur among the termites, ants and bees, but are known especially among the gall-flies (Cynipidæ); indeed, one entire division, comprising more than 500 species, is named

Inquilinæ, because of the predominance of this mode of life. These insects differ but little in structure from the true gall-flies, but they lack the power to produce galls and consequently deposit their eggs within those of other species. They infest certain species of galls, as those of the blackberry and some oak-galls, in large numbers and sometimes more than one kind occur in a single gall. Perhaps the most remarkable feature of these inquilines is their frequent close resemblance to the insect which produces the gall which they infest.


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Rays of Positive Electricity and Their Application to Chemical Analyses/Evolution of Helium and Neon

I have examined by the positive ray method the gases given off by a great variety of substances when they are bombarded by cathode rays,—the substances include most of the metals, a considerable number of minerals, and many metallic salts,— and have found in almost every case when cathode rays had a high speed that traces of helium were present in the gases liberated in this way. The rate of evolution of helium is generally considerably greater at the commencement of the bombardment than when this has been prolonged for some time, but in the majority of cases there seems to be a residual effect which remains even when the bombardment has lasted for a long time. The larger effect observed at first is due I think to helium absorbed by, or accumulated on, the substance bombarded; the question arises, is the more lasting evolution of this gas due to the liberation of accumulated helium lingering in the solid, or are the atoms of the elements bombarded by the cathode rays disintegrated, one of the products of disintegration being helium, or as a third alternative are there salts of helium present in the substance as impurities, and are these dissociated by the cathode rays. The first published account of the evolution of helium in vacuum tubes is that given by Sir William Ramsay ("Nature/1 July, 18, 1912); he found that when the glass of old Rontgen ray bulbs was heated, sufficient helium was given off to be detected by spectroscopic methods.

Before proceeding to consider how the various explanations of the appearance of helium may be tested, it is advisable to discuss some general aspects of the question. The difficulty of finding the origin of the helium is considerably greater than the corresponding problem for X$3$ owing to the presence of helium in the atmosphere. The positive ray method is so sensitive that the amount of helium in a cubic centimetre of air (which according to Ramsay is about 4 x 10$-6$ c.c at standard temperature and pressure) produces a strong line on the photograph, a line stronger than that found in most of the experiments described below, so that in the greater part of these experiments we are dealing with amounts of helium less than that which could be accounted for by the entrance of 1 c.c. of air into the apparatus. This source of helium has to be carefully guarded against; it is necessary, for example, to be very careful in the use of charcoal cooled by liquid air for producing the final vacuum. The cooled charcoal hardly absorbs the helium at all and thus this gas is not taken out of the vessel by this method of exhaustion. For example, if the first stages of the exhaustion were done by a water pump which takes the pressure down to a centimetre or so, and the rest of the exhaustion done by cooled charcoal, -sufficient helium and neon would be left in the vessel to give very strong lines on the positive ray photograph. It is necessary in experiments of this kind to reduce by a mercury pump the pressure to a fraction of a millimetre of mercury before applying the cooled charcoal.

We can, however, in experiments when the helium is liberated by prolonged bombardment eliminate this source of error, for if the helium and neon came from the air and not from the solid the amount of them produced would not depend on the length of the bombardment. I have checked In this way these experiments and have found that the helium is not appreciable unless the bombardment Is prolonged for an hour or so and increases in amount with the length of the bombardment, Thus if the helium comes from the air the air must have been absorbed by the substance and the helium in It liberated by the bombardment.

To test whether this was the source of the helium I bombarded soluble salts such as LiCl, NaCl, KCl, KI, RbCl, Ag NO$3$ which were dissolved in water and also in some cases in alcohol and then evaporated to dryness, the process being in some cases repeated several times. Salts which had been treated In this way yielded helium and in some cases neon; the yield of helium from the salts of the alkali metals and in articular from potassium was exceptionally large, KI giving a larger supply than any other of the substances I examined, with the exception of those like monazite sand which are known to contain large supplies of helium. Some of the salts have yielded apparently undiminished supplies of helium, after being dissolved and evaporated ten or twelve times. In order to see whether this process of solution and evaporation would get rid of dissolved helium, the following experiment was tried. It is well known that when the electric discharge passes through helium from aluminium electrodes, these electrodes absorb a considerable amount of helium. A piece of aluminium was divided into two portions, one half was made into electrodes of a vacuum tube filled with helium at the pressure of three or four millimetres of mercury and a current passed through the gas for two days, after this treatment the electrodes were dissolved in hydrochloric acid and evaporated to dryness. The salt thus obtained was then placed in the positive ray apparatus, bombarded by cathode rays for several hours, and a positive ray photograph of the gas given off was taken; it was found that the helium line was faint but perceptible. The other half of the aluminium which had not been near helium was then dissolved, evaporated, bombarded and the photograph taken, the intensity of the helium line in this case was but very little less than in the other, the difference not being greater than one would expect from accidental variations in the intensity of the discharge; this experiment shows, I think, that solution may be relied upon to eliminate absorbed gas.

The aluminum cathode in the tube used to bombard the substances with cathode rays might be suspected as a source of helium. If this were the case, however, the rate of production of helium would not depend upon the nature of the salt bombarded, nor would it make any difference as to whether the cathode rays hit the salt or not. As both these conditions have a great influence on the rate of production of helium we may regard this source as eliminated. In addition to the preceding considerations some of the cathodes have been in almost continuous use for months without any perceptible diminution In the rate of supply of helium.

There Is another possibility which It Is much more difficult to eliminate by means of physical experiments, though the chemical properties of the inert gases may be thought to make It very improbable; this is that helium can combine with a large number of elements and that these helium compounds exist as impurities In the salts of these elements and that these compounds of helium, present as impurities in soluble salts, are also soluble. The fact that helium is given off by nearly every substance shows that this solid compound or compounds must be extremely widely spread if it is to explain the helium production. They must also possess very special chemical properties in order to explain a large number of cases of which the following is an example: if we take AgNO$3$, a salt which will give some helium on bombardment, dissolve it in water, add HCl, we get a precipate of AgCl, now this silver chloride on bombardment will also give helium.

Thus if compounds of helium are present as impurities they must be precipitated by the same chemical reactions as precipitate the salts of the element with which they are mixed. It must I think be acknowledged that to explain the production of helium by the dissociation of helium compounds present as impurities obliges us to assign to helium chemical properties of a much more energetic kind than those usually assigned to it. On the other hand there are some effects connected with the appearance of helium which suggest that the source of this gas is not the whole mass of the salt but only a small fraction of it, and which would in fact be more easily explained by the presence of an impurity than on the view that the cathode rays can detach helium from any atom, of potassium say, which they happened to bombard. One effect of this kind is the very considerable variation in the amount of helium which comes off from different specimens of the same salt when bombarded under apparently similar conditions, and sometimes also of considerable variations In different salts of the same metal; thus I have always got more helium from KI than from KCl. We must not, however, lay too great stress on this for it is difficult to ensure that the bombardment by the cathode rays is of the same intensity in any two experiments; a very slight variation of the pressure of the gas in the bombardment vessel might produce large variations In the energy of the rays striking against the salt.

Another effect, which also favours the impurity view, is that the rule of evolution of helium, unlike that of X$3$ by the bombardment of potash, in nearly every case shows a tendency to diminish after long continued bombardment. This occurs even with salts which have been dissolved and evaporated to dryness. Experiments which are not yet completed are being made to see if the evolution of helium stops entirely after long continued bombardment. These effects could be explained and the difficulties attendant on the view that the helium comes from an impurity of the ordinary kind avoided by some such view as the following. The evolution of helium is not due solely to the bombardment, by the cathode rays. In the atoms of the ordinary elements and especially in those of the alkali metals a process is at work analogous to that which causes the expulsion of a particles from the atoms of the ordinary radioactive elements; the difference being that In the case of the ordinary elements the a particle, i.e. the helium, instead of being projected with the enormous velocity characteristic of radio-active substances, is projected with so little energy that It does not wholly escape from the parent atom. It Is loosened, so to speak, by effects which are analogous to radio-active effects and are independent of the cathode rays, the function of these rays is to detach the already loosened helium atoms. Thus it would only be certain atoms of the element which would yield helium when bombarded and when these were exhausted the supply of this gas would cease. The number of such atoms too might be expected to vary with different specimens of the same salt. Another view that might be suggested is that the helium might be formed in some such way as we have supposed X$3$ to be formed, i.e. by the aggregation of atoms of hydrogen. If this were the case, however, we should expect that the formation of helium would be much more pronounced when the bombarded salts contained hydrogen than when they did not; as a matter of fact, however, some of the salts which yield the largest supply of helium such as KI do not contain any hydrogen.

The view thiat helium can be got from other chemical elements raises questions of such a fundamental character that few will be prepared to accept it until every other explanation has been shown to be untenable. It would greatly strengthen the proof if we could detect the parts of the atom which remain when the helium is split off. Experiments are being made with this object but there are very considerable difficulties to be overcome. The alkali metals have so far afforded the largest supply of helium; let us consider what the residues would be in this case. If we took He from Li the residue would probably be hydrogen, now hydrogen always occurs in the positive rays and no conclusion could be drawn from its occurrence when lithium was bombarded. Next take the case of sodium; the residue after the abstraction of helium would have the atomic weight 23 - 4 = I9, which corresponds.to fluorine, an element of such energetic chemical properties that it would probably enter into chemical combination and escape detection; similar considerations apply to potassium, the residue from which might be chlorine—this again on account of Its chemical properties would be difficult to detect. If we were to take elements of much greater atomic weight the residues would have high atomic weights also and these heavy atoms are not detected by the photographic plate nearly so easily as the lighter ones like helium, so that even if there were the same number of the atoms of the residue as of the helium present In the tube the helium might show on the plate while the residues did not. The most promising elements for this purpose are light elements like carbon or beryllium whose residues would not coincide with any likely Impurities in the tube. I have tried some experiments with pure carbon but hitherto have only been able to obtain such minute quantities of helium from it that it is impossible to draw any definite conclusions.