text:
Rays of Positive Electricity and Their Application to Chemical Analyses/Effect at Very Low Pressures

When the pressure Is reduced to as low a value as is possible the appearance of the luminosity on the screen entirely changes. At these low pressures It Is exceedingly difficult to get the discharge  to   pass  through   tubes  of moderate size when the cathodes are made of aluminium or any of the metals ordinarily used for this purpose, and there Is great danger of sparks passing through the glass and breaking the tube. This can be avoided to a great extent by facing the cathode with a thin layer of calcium, or smearing the face of the cathode with the liquid alloy of sodium and potassium. This reduces considerably the difficulty of getting the discharge to pass and diminishes the risk of perforating the tube. The appearance at these low pressures when hydrogen or air is In the tube Is shown In Fig. 9. It will be noticed that the straight  bands  of  phosphorescence   have almost   disappeared    and   that   most   of phosphorescent light Is concentrated Into two parabolic curves which are connected with the undeflected spot by straight faintly luminous lines. The value of e/m for one parabola is 10$4$, that for the other 5 x 10$3$ so that  they are  due  to  the  atom  and  molecule of  hydrogen respectively. At these low pressures the luminosity In the negative direction disappears. But both at the low and higher pressure there is, even when the magnetic and electric fields are in action, an appreciable amount of luminosity at the position occupied by the undeflected spot.

There is considerable advantage in using very large glass vessels for the discharge tubes when studying positive rays; with large vessels the pressure can be made very small before the tube offers great resistance to the passage of the discharge through It The increase in the difficulty of getting the discharge to pass comes in at the pressure when the dark space round the cathode reaches the walls of the tube. When the tube is big the walls are far away from the cathode and the pressure has to be exceedingly low before the dark space reaches the sides of the tube. We can work with much lower pressures with these large tubes and therefore reduce the obstruction which the positive rays meet with in their passage from the cathode to the screen. Using vessels of about 2 litres capacity I have observed$1$ on a willemite screen the parabolas corresponding to carbon, oxygen, neon, and mercury vapour as well as those corresponding to the atom and molecule of hydrogen and the atom of helium. The photographic plate is, however, for most purposes a much more convenient detector than a willemite screen. It is more sensitive, it gives a permanent record, and measurements can be made with much greater accuracy on the plate than they can on the screen. Before entering into the discussion of the theory of the positive rays it is desirable to describe the results obtained with the photographic method, as well as the experimental details by which these results have been procured.

The apparatus now in use at the Cavendish Laboratory is represented in Fig. 10. The discharge takes place in a large glass flask A: a volume of from one to two litres is a convenient size for this purpose. The cathode C is placed In the neck of the flask. The position of the front of the cathode has a very considerable influence on the brightness of the positive rays and ought to be carefully attended to. The best position seems to be when the front of the cathode is flush with the prolong tion of the wider portion of the flask. The shape of the cathode is represented in section in Fig. 11: the face of the cathode is made of aluminium, the other portion is soft iron. A hole is bored right through the cathode to admit the fine tube through which the positive rays are to pass. Care should be taken to bore this hole so that its axis is the axis of symmetry of the cathode. The tube through which the positive rays pass is fastened into the cathode in the way shown in Fig. 11.

The bore of this tube will vary with the object of the experiment If very accurate measurements are required, the diameter of the tube must be reduced to 1 mm. or less. With these very fine tubes, however, very long exposures (1½ to 2 hours) are necessary. The length of the tube is about 7 cm. The tubes are prepared by drawing out very fine bore copper tubing until the bore is reduced to the desired size. The tube is straightened by rolling it between two plane surfaces, and great care must be taken to get the tube accurately straight, as the most frequent cause of dimness in the positive rays is the crookedness of the tube. After long use the end of the tube nearest the discharge tube gets pulverized by the impact of the positive rays, and the metallic dust sometimes silts up the tube and prevents the rays getting through. The cathode is fastened In the glass vessel by a little sealing-wax, and a similar joint unites it to the ebonite box, UV. To keep the joints cool and prevent any vapour coming from the wax, the joints are surrounded by a water jacket J through which a stream of cold water circulates.

The electric field is produced between the faces of L and M which are pieces of soft Iron with plane faces. These are fitted Into the ebonite box UV so that their faces are parallel: the distance between the faces should be small compared with their lengths. In many of the experiments described subsequently the length of the faces was 3 cm. and their distance apart 1.5 mm. Their faces are connected with the terminals of a battery of small storage cells: In this way any required differ- ence of potential can be maintained between them.

These pieces of soft Iron practically form the poles of an electromagnetic, for the poles of the electromagnet P and Q are made of soft Iron of the same cross section as L,M ; they fit into Indentations In the outside of the ebonite box and are only separated from the pieces L,M, by the thin flat pieces of ebonite which form the walls of the box. This arrangement makes the magnetic field as nearly coterminous as possible with the electric, which Is desirable in several of the experiments. A conical glass vessel F 40 cm. long is fastened by wax to the ebonite box while the other end is fixed to the apparatus which contains the photographic plate. One form of this, designed by Mr. Aston, Is represented in Fig. 12. The photographic plate Is suspended by a silk thread wound round a tap T which fits into a ground glass joint; by turning the tap the thread can be rolled or unrolled and the plate lifted up or let down. The plate slides in a vertical box B made of thin metal; this Is light tight except at the openings A which are placed so that the positive rays can pass through them. The openings are on both sides of the box and about 5 cm. in diameter. When the silk thread is wound up the strip DEFG of photographic plate in the box Is above the opening A, so that there Is a free way for the rays to pass through A and fall on a willemite screen behind It   This  screen is not used    for   purposes    of measurement,  but  only to see before taking the photograph that the tube Is   giving   an adequate supply of positive rays. The box Is sufficiently large to hold a film long enough for two or more photographs;   If   It   Is wished  to  take  two photographs,   the   plate Is lowered until the bottom half comes opposite to   the   opening   A,   a photograph Is taken In this position,  the plate Is   then  let  down   still further until the top half of the plate comes opposite to the opening, then a second photograph Is taken. This plan Is convenient because the deflections of the different kinds of positive rays differ so much that it is difficult to measure them accurately when they are all on one plate, For example the magnetic deflection of the hydrogen atoms Is about fourteen times that of the mercury one, thus If the deflection of the hydrogen atom is within the limits of the plate, that of the mercury atom would be too small to measure accurately. When we can take two photographs, however, without opening the tube, we may take one with a small magnetic fleld to get the deflection of the hydrogen atom, and the second with a much larger one to get the deflection of the mercury one.

Two tubes containing coconut charcoal are fused to this part of the apparatus; by immersing these in liquid air the pressure can be made exceedingly small. As the only communication between this part of the apparatus and that through which the discharge passes is through the long and very narrow tube in the cathode, it is possible to have the pressure on the camera side of the apparatus very much less than the pressure on the side through which the discharge is passing.

A Gaede pump worked by a motor is connected with the discharge tube, and keeps the pressure in this part of the apparatus at a suitable value. When the rays in some particular gas are under examination a constant stream of this gas is kept flowing through the discharge tube. The gas is stored in the vessel A, Fig. 13, over a column of mercury: this vessel is connected with the discharge tube by the system shown in Fig. 13, where EC is an exceedingly fine capillary tube. When the tap T is turned the gas has to pass through this capillary: it does so exceedingly slowly. The rate can be adjusted by raising or lowering a mercury reservoir connected with A, this is held in such a position that when the Gaede pump is in action the pressure in the discharge tube is such as to give well developed positive rays. To screen off the magnetic field due to the electromagnet, thick iron plates V,W, Fig. 10, are placed round the neck of the tube.

The curves on the photographic plates made by the positive particles are measured by the apparatus represented in Fig. 14. The photographic plate is clamped in a holder A, and the position of any point on it is determined by moving the carrier C until the tip of the needle NN comes just over the point in question. The carrier C has two movements, one parallel to the base BB, and the other, by means of the screw S, at right angles to this direction, the position of the point is read off on the two verniers. The plate is placed In the holder so that the direction of the magnetic  deflection Is parallel and that of the electrostatic deflection, at right angles to BB.

$1$ J. J. Thomson, " PML Mag.," VI, xx, p. 752, 1910.


text:
Rays of Positive Electricity and Their Application to Chemical Analyses/Discussion of the Photographs

The appearance of a typical photograph produced by the Impact of the cathode rays on the plate when the pressure on the camera side of the apparatus Is reduced to about .001 mm. of mercury Is shown in Fig. 15, Plate I. In this and the figures the deflection due to the magnetic field is vertical, while that due to the electrostatic field is horizontal. It will be seen that the curves on the plate are of two different types.

I. A series of separate parabolic arcs, often of considerable length. From the theory given on page 13 it will be seen that each of these parabolas arises from particles having the same value of e/m, and that these particles have retained this charge throughout the whole of the journey through the electric and magnetic fields. As the velocity of a particle is by equation (3), p. 12 proportional to the tangent of the angle which the line joining the origin to the point where the particle hits the screen makes with the horizontal, It follows that there is a considerable range of velocities among the particles having the same value of e/m. In many cases we have velocities among the same kind of particles differing as much as to make the velocity of the slowest ones less than one fifth that of the fastest In some cases the parabolas are of fairly uniform intensity along the whole of their length. In others as In that shown in Fig. 16, Plate I, the head of the parabola (the part least deflected) is considerably brighter than the rest of the curve, while sometimes, as in the case represented in Fig. 17, Plate L, there are several spots of maximum luminosity dotted along the parabolic arc.

With some exceptions (to be considered later) the heads of all the parabolas are in the same vertical line, showing that the minimum electrostatic deflection suffered by the particles which produce these curves is the same for all the different kinds of particles. By equation (2) page 12 the electrostatic deflection is proportional to e/mv$2$. If the energy of the particles is due to the fall of the charge through a potential difference V

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

so that

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

Hence as the minimum electrostatic deflection is the same for all the particles, we conclude that the maximum potential through which the various particles have fallen is the same for all particles. It is natural to conclude that this maximum potential is the difference of potential between the anode and cathode of the discharge tube. It is easy to verify that when the pressure is altered so as to increase this difference of potential)the deflection of the heads of the parabolas diminishes.

2. Besides the parabolas there are on the plate a series of straight lines connecting the parabolas with the origin. These are due, I think, to particles which have been charged during a part only of their passage through the electric and magnetic fields. This might happen in two ways. A particle which had got neutralized before reaching these fields might, while passing through them, come into collison with a corpuscle, get ionized, and acquire a positive charge, and during the rest of its journey be deflected by the electric and magnetic forces. Or again a particle might be positively charged when it entered the fields,  attract  a corpuscle whilst   in them,  get neutralized and for the rest of its journey be free from electric and magnetic deflections. This view of the origin of these to me to be proved by the following experiments,.

As on this view these lines are due to particles which are or discharged in the electric and magnetic fields ; their intensity, as compared with that of*the parabolas, ought to diminish If the length of these fields Is reduced. To test this 1 took a photograph with a tube when the lengths of the electric and magnetic fields were reduced to I mm., the in- tensity of the fields being Increased In proportion so as to get deflections comparable with those in the longer fields. With this very short field the straight lines disappeared, and nothing except the parabolas and the undeflected central spot was to be seen on the photographic plate.

Another way of testing this view Is to use magnetic and electric fields, which are not coterminous. Let us suppose for example that the magnetic field stretches beyond the electric, on the camera side. There will be a part of the field where the particles are exposed to magnetic but not to electric forces. If a neutralized particle gets Ionized in this region, it will experience magnetic, i.e. vertical deflection but no electrostatic or horizontal deflection. Thus with a field of this kind we should expect the line due to particles which acquired their charge whilst in the electric field to have the shape shown In Fig. 18. The straight vertical stem near the origin Is due to the particles Ionized beyond the electric field, a piece running up to join the parabola to those ionized Inside this field, the portion close to the parabola being due to particles which get Ionized almost as soon as they enter the fields. Photographs taken with the magnetic field overlapping the electrostatic show this effect very plainly; one of them is reproduced in Fig. 19, Plate L, another In Fig. 25, Plate II.

Let us now consider the case of the charged particles which get neutralized while passing through the field. The part of the line near the origin will be due to particles which get neutralised almost as soon as they enter the field. We have supposed that the magnet was moved towards the camera so that its field overlapped the electric on that side. This will tend to make the electric field overlap the magnetic on the other side, i.e. the side nearest the cathode, so that when a particle first enters the field Its deflection is mainly due to the electrostatic force and is therefore horizontal; thus a particle which gets neutralized at the early stages of Its journey through the fields will have a horizontal displacement abnormally large compared with the vertical; so that the curves produced on the photographic plate by the particles which get neutralized will have a shape something like that shown in Fig. 20. We see that with these overlapping fields we can distinguish between the lines which are due to particles which have gained a charge in their journey and those which have lost one. The concavities of the two curves are in opposite directions. These two sets of lines are very prominent In photographs taken with apparatus in which care has not been taken to make the fields coterminous; an example of this Is shown in Fig. 19, Plate I. If the fields are coterminous and uniform the  two curves coincide and are straight lines passing through the origin.

The gain or loss of the charge on the particles in  the positive rays is shown very directly by the following experiments:$1$ The positive rays were produced in a tube made so as to allow room for two electromagnets A and B, Fig. 21, to be inserted between the cathode C and the willemite screen S. The magnets were placed so that the magnetic field due to the one nearest the cathode was horizontal and the deflection due to It therefore vertical, while the field due to the magnet next the screen was vertical and the deflection due to it horizontal.

The deflections due to the two magnets could thus be separated and measured independently. The effects observed when the magnets were applied separately and then In succession are Interesting. A typical case Is represented In Fig. 22.

Fig. 22 (a) represents the appearance of the screen when the electromagnet next the cathode Is the only one in action: a Is the position of the undeflected spot, b that of the deflected. a and b are connected together by a straight band of luminosity; the luminous streak above a Is due to negatively charged particles. Fig. 22 (b) represents the appearance when both magnets are on. If there had been no loss or gain of charge the only effect of the second field would be to remove the spot b horizontally to another place b’, and we should have only two spots visible a and b’. We see that as a matter of fact there are four spots a, a’, b, b’ on the screen, as well as considerable luminosity over the rectangle with these points as corners. Let us consider these spots in succession : b' has experienced the full horizontal as well as the full vertical deflection : it is therefore produced by particles which have retained their charge whilst passing through both magnetic fields. Let us now take b: this spot has the maximum vertical deflection but no horizontal deflection. Thus the particles producing this spot must have been charged all the time they were in the field of the electromagnet A, but have lost their charge before reaching the field of the electromagnet B. This is an example of a particle losing a charge on its way down the tuba Now consider the spot a’: this has not been deflected vertically at all, therefore it must be due to particles which were uncharged when they were passing the first magnet A. On the other hand it has experienced the full horizontal deflection, showing that the particle most have acquired a charge before reaching the second magnet B, This is an example of particle acquiring a charge during its path. The appearance of the luminosity due to the negatively charged particles shows that these, too, gain and lose negative charges in their passage down the tube.

When we reduce the pressure to the lowest value we can reach by the use of charcoal and liquid air, then with the magnet A on alone we have the spots a and b, Fig. 22. There Is no luminosity between them and no luminosity above a, while, when both magnets are on, we have merely the spots a and b’ ; b and a’ have disappeared along with the luminosity inside the rectangle.

We shall call the lines we have just been considering secondary lines, the parabolic arcs primary lines.

It is important to point out that the collision which Ionizes a neutral particle and gives it a positive charge must be a collision with a corpuscle and not with a molecule of the gas through which the positive rays are passing; for the mass of a molecule of the gas is comparable with that of the positive ray particle, hence a collision between the two would result in the particle losing an appreciable fraction of its energy and being deflected through a considerable angle. The appearance and inclination of the secondary lines show that the particles suffer little diminution in velocity in these encounters and no appreciable change in direction, hence we conclude that the system with which the particle collides must have a much smaller mass than the particle, i.e. it must be a corpuscle and not a molecule.

The secondary curves finally join the parabolic arcs produced by the particles which have been charged during the whole of their journey. If the junction occurs at a considerable distance from the head of the primary, care has to be taken in some cases to avoid confusing the secondaries with primaries corresponding to a different value of ejm. Thus, for example, if the shape of the secondary and primary were similar to that shown in Fig. 23a, and the point of junction came off the plate, the appearance on the plate would be that represented in Fig. 23b, and the secondary might be mistaken for a primary with a value of e\m less than the true value. If the magnetic field overlapped the electric field on the camera side of the apparatus, the primary and secondary would resemble Fig. 23a, and if the right hand part were off the plate, the curves would look like Fig. 24b and the secondary might be mistaken for a primary with a value of e/m greater than the true value. This possible confusion of a secondary with a primary line is a point which requires careful attention when the curves produced by the positive rays are used to identify the gases in the discharge tube; for this purpose the primary curves are the only ones that can be relied upon. The tests for a primary line are (1) that it is parabolic, (2) that it shows an abrupt increase in intensity at a point in the same vertical line as the heads of the other parabolas. The first condition is theoretically sufficient, but when only short arcs are available, it is often difficult, unless a very high degree of accuracy is obtained In the measurement of these lines, to tell whether the curve is or Is not a parabola.

A very interesting feature about these secondary lines is that the velocity of the particles which produce them is practically independent of the strength of the electric field in the discharge tube. When, as a result of a change in pressure, the potential difference between the anode and cathode increases, the velocity of the particles which produce the primary curves increases also. The speed of the particles which give rise to the secondary curves on the other hand is little if at all affected. A little consideration will show, however, that this is what we might expect from the theory given on page 28. The secondaries are supposed to be due to particles which have gained or lost a charge whilst passing through the electric and magnetic fields. Let us consider firstly the case of the particles which have gained a charge: they must have done so by coming into collision when moving at a high speed with a corpuscle which, as it is in the space behind the cathode where there is no electric field, will be approximately at rest. The effects of the collision will, however, clearly depend only on the relative motion of the particle and corpuscle, and will be the same as if the particle were at rest and the corpuscle moving with the velocity of the particle. Now in order that a moving corpuscle may ionize an atom or molecule against which It strikes, the velocity of the corpuscle must exceed a certain value which recent researches$2$ show depends to some extent on the nature of the atom or molecule. Let us call this limiting velocity for a particular kind of atom V; then in order that an uncharged atom of this kind should be ionized when it strikes a corpuscle at rest it must move with a velocity greater than V; hence all the secondary rays of this kind formed by these atoms must have a velocity greater than V. There is thus an inferior limit to the velocity of the secondary rays and this limit depends on the kind of atom which produces these rays. There must, however, be a superior limit to this velocity as well as an inferior one, for these rays are due to particles which move with great velocity and yet can have lost their charge. To have acquired this velocity they must have been positively charged before passing through the cathode or they would not have been acted upon by the electric forces in the discharge tube: and as they are uncharged when they reach the magnetic field they must have got neutralized by combining with a negative corpuscle in the interval. Now a positively electrified particle moving rapidly past a corpuscle could not attract and hold fast the corpuscle if the relative velocity of the particle and corpuscle exceeded a certain value. This velocity is evidently determined by the condition that it is the velocity with which a corpuscle must be moving when it has just sufficient energy to escape from the surface of a positively electrified particle at rest. We should expect the work required to separate a corpuscle from the surface of a positively electrified particle to be of the same order as that required to ionize the particle when neutral, and this work is equal to the kinetic energy of a particle moving with a velocity V. Thus if the velocity of the particle were appreciably greater than V a positively charged particle would not get neutralized, while if the velocity of the neutral particle were appreciably less than V it would not get ionized. Hence the velocity of the secondaries we are considering must be very approximately equal to V, a velocity which depends only on the nature of the particle and not on the potential difference applied to the discharged tube. This accounts for the fact that the velocities of the particles forming the secondary rays are independent of the potential difference between the anode and cathode.

We can by the method described on page 12 measure the velocity of the particles In the secondary rays corresponding to any atom and hence determine V, the smallest velocity which a corpuscle can have if it is to be able to ionize the atom. When this method Is applied to the secondary rays connected with the hydrogen atom we find that V is about 2 x 10$8$ cm./sec. This velocity would be acquired by a corpuscle if it fell through a potential difference of 11 volts. Hence we may take 11 volts as the measure of the energy required to ionize an atom of hydrogen.

To give to the atom of hydrogen this velocity requires a potential difference of 11 x 1.78 x 10$7$/10$4$ volts (taking e/m for the corpuscle to be 1.78 x 10$7$ and for the atom 10$4$), this Is about 20,000 volts. If it required the same energy to ionize an atom of oxygen as one of hydrogen, V would be the same for oxygen as for hydrogen. To give an atom of oxygen this velocity would require a potential difference of 16 x 20,000, or 320,000 volts, a much greater potential difference than we usually apply to the discharge tubes. Thus we see that we cannot expect any except the lighter gases such as hydrogen or helium to show secondaries of the type we are considering.

There is, however, another type of secondary—that due to particles which enter the magnetic field in a charged state and lose their charge before emerging from it: this type of secondary, since it arises from the combination of a positively charged particle with a negatively charged corpuscle, might be expected to occur with slowly moving particles more readily than with fast ones; a particle moving faster than a certain speed would not combine with a negative corpuscle, so that there would be a superior limit to the speed of the particles in secondary rays of this kind. There does not, however, seem to be any reason why there should be an inferior limit to the velocity, provided the slow particles have managed to retain their charges up to the beginning of the magnetic field, and as a matter of fact this type of secondary often shows itself, as in Fig. 25, Plate II., as the limit of a patch of fogging on the photographic plate rather than as a sharply defined line. There are, however, cases notable with the mercury lines, when this type of secondary is more sharply defined than we should expect, since there are among the particles which produce the primary parabolas some with a smaller velocity than can be detected In the secondaries of this type.

The question arises whether the corpuscles which produce the secondaries by neutralizing a positively charged particle or ionizing a neutral one are free, or are those bound up in the molecules of the gas through which the positive rays are travelling. There are several reasons for thinking that the latter hypothesis is the more probable one.

For if the corpuscles which neutralize the positive particles are free they should be removed by a strong electric field which ought therefore to dimmish the brightness of the secondaries. I have, however, never been able to detect an effect of this kind.

Again if free corpuscles were those which neutralized the positively charged particles, the distance such a particle would travel before it got neutralized would depend only upon the density of the free corpuscles. Now this density depends upon the amount of ionization produced by the positive rays after they have passed through the cathode; this will vary with the number of these rays as well as with the pressure and nature of the gas through which they travel. Consequently the distance a positive particle has to travel before it gets neutralized will depend upon other things besides the pressure and character of the gas, and will not therefore have a very close connexion with the free path of a molecule of this gas. Wien$3$ who has made a very complete study of the distance a charged particle travels before it gets neutralized finds that it is of the same order of magnitude as this free path and does not depend upon the number of free corpuscles. This is in favour of the view that the corpuscles which neutralize the positive particles are not free, the process of neutralization seems to be that the positive particles move through the molecules of the gas and pluck out of them the corpuscles required for neutralization.

Again, we conclude for similar reasons that the corpuscles which ionize a neutral particle are not free but bound up in the molecules of the gas through which the positive rays pass. These rays, like the a particles, seern to be able to pass right through molecules, and Königsberger and Kutschewski$2$ have shown that when moving through a gas they suffer little diminution in velocity until they are nearly at the end of their path. If the corpuscles which neutralize the positive particles are not free but are in the molecules of the gas through which these particles are passing, we can understand why there is a lower as well as an upper limit to the velocity of the particles which give rise to the secondaries due to particles which have been neutralized while passing through the electric and magnetic fields. For on this view the particle before it can be neutralized has to detach a corpuscle from an atom or molecule. To do this requires the expenditure of a definite amount of work which has to be done on the corpuscle by the particle. The energy communicated to the corpuscle depends on the velocity of the particle, and unless this velocity reaches a definite value the corpuscle will not get enough energy to escape from the molecule and will thus be unable to neutralize the particle.

$1$ J. J. Thomson, "Phil. Mag.," VI, XVIII, p. 824, 1910.

$2$ Franck and Hertz, " Verhand. d. D. Phys. Ges.," 15, 34, 1913.

$3$ W. Wien, Berlin, " Sitzraigsberidite," July, 1911.

$4$ Königsberger and Kutsdiewski, Heidelberg, "Sitzungsberichte," Jan., 1912; "Ann. der Phys.," 38, p. 161.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Rays of Positive Electricity and Their Application to Chemical Analyses/Negatively Charged Particles

We have already seen (p. 18) that besides the particles which cany a positive charge of electricity there are others which carry a negative one. These negatively charged particles show many analogies with the particles which produce the secondary rays we have been considering. Like them they are particles which have changed their condition after passing through the cathode. Before passing through the cathode they were positively charged and they owe the high velocity they possess to the action on this charge of the electric field in front of the cathode. After passing through the cathode they get neutralized and then attract to themselves a negatively electrified corpuscle which gives them a negative charge. The attraction which brings the corpuscle and the particle together is the attraction between a neutral particle and a corpuscle. We may imagine that this attraction is the result of electrostatic induction between the charge on the corpuscle and the particle. The magnitude of this attraction will depend upon how nearly the particle behaves like a conductor of electricity, or perhaps more accurately like a body of very great specific inductive capacity. The greater the specific inductive capacity the greater the attraction, while if the specific inductive capacity Is the same as that of the surrounding medium there will be no attraction at all. It is not surprising therefore to find that different kinds of atoms and molecules differ very greatly In their power of acquiring a negative charge. With two or three exceptions to be   mentioned   later  I   have never found molecules with with a negative charge, though molecules positive charges are quite common. The exceptions are the molecules of oxygen, carbon, and, though very rarely, hydrogen, and the hydroxyl radicle. It will be noticed that these are elements or radicles, and I have not met with any case when the molecule of a compound has been found with a negative charge, though they are found readily enough with positive charges. I do not mean by that it is impossible to give a negative charge to a molecule of a compound gas when it is electrically neutral, but merely that the attraction between such a molecule and a negatively electrified corpuscle is so feeble that it is not sufficient to enable the molecule to keep a permanent hold on the corpuscle when it sweeps past it with the velocity with which these molecules move in experiments with the positive rays.

There are some atoms also which I have never observed with a negative charge even when large quantities of them with positive charges were coming through the hole in the cathode. The atoms of helium, nitrogen, neon, argon, krypton, xenon, and mercury belong to this type. On the other hand negatively charged atoms of hydrogen, carbon, and oxygen are found in almost every tube where the pressure is not too low, the negatively charged oxygen atom being specially prominent If chlorine or any of its gaseous compounds are present in the tube, negatively charged chlorine atoms are conspicuous. The brightness of the lines due to the negatively charged particles, as compared with that of those due to the positive ones, increase rapidly as the pressure increases. At fairly high pressures I have seen the lines due to negatively charged hydrogen atoms quite comparable'in intensity with those due to the positive ones.

Though in one sense all the lines due to negatively charged particles are secondaries they show differences amongst themselves corresponding to the difference between the primary and secondary positive lines. Some of the negative lines like the positive secondaries come close up to the origin, while there are others which, like the primary positives, are finite arcs of parabolas, terminating abruptly when they approach within a certain distance of the vertical axis. Indeed the lines on the negative side are frequently exact reproductions in shape and size of the corresponding lines on the positive. An example of this is shown in Fig. 26, Plate II., where the curves α and β are the lines corresponding respectively to the positively and negatively electrified atoms of oxygen when the discharge passed through very pure oxygen: it will be seen that every detail in the positive curve is reproduced in the negative. This might suggest that the positive and negative atoms were the two halves of a neutral molecule which divided after passing through the cathode. Further consideration, however, shows that this view is not tenable, at any rate in the great majority of cases. The heads of the negative parabolas, like those of the positive, are all on a vertical line and the distance of this line from the vertical line through the origin is about the same as the corresponding distance for the positive parabolas. From this it follows by equation (2) (p. 12) that the maximum value of ½mv$2$/e is the same for the negative as for the positive particles and equal to the potential difference between the anode and cathode of the discharge tube. To take a definite case, let us suppose that the negatively charged hydrogen atom owes its charge to having been in chemical combination with an atom of carbon before it passed through the cathode, the molecule of the compound being positively charged when in the discharge tube and acquiring a high velocity under the electric field. After passing through the cathode the molecule gets neutralized and then dissociates into a positively charged carbon atom and a negatively charged hydrogen one. The kinetic energy acquired by the molecule CH, If it had one charge of electricity,would be measured by V, the potential difference between the anode and cathode in the discharge tube. Since the mass of the carbon atom is twelve times that of the hydrogen one, the kinetic energy of the hydrogen atom would correspond to a fall of potential V/13, so that if this atom went through the electric and magnetic fields as the positively charged carbon atom, the horizontal deflection of the hydrogen atom would be twelve times that of the carbon one. The photographs show, however, that the deflections are equal. Again if the negatively electrified oxygen atoms had previously been in combination with an atom of hydrogen we should expect that the addition of hydrogen to pure oxygen would increase the brilliancy of the lines due to negatively electrified oxygen. I have never been able to detect any effect of this kind. It is true that when some compounds of oxygen are in the tube the negative lines are brighter than with other compounds. I am inclined to think that the exceptional brightness of the lines is due to the presence of the hydroxyl radicle and that these exceptionally bright lines are OH and not O. The view which seems to accord best with the observations is that given on page 40, that the negatively electrified atoms are atoms which were positively electrified when in the discharge tube, that they got neutralized after passing through the cathode by combining with a corpuscle, and in this neutral condition exerted so strong an attraction upon a corpuscle that they were able to capture it though moving past it with an exceedingly high velocity.

Taking this view, we can form an estimate of the magnitude of the attraction between a neutral atom and a corpuscle. From the measurement of the plates we find that there are negatively electrified atoms of hydrogen with a velocity as large as 2 x 10$8$ cm/sec. This means that a neutral atom of hydrogen is able to capture a corpuscle even though it is moving past it with this velocity. This capture, however, would not take place unless the work required to remove a corpuscle from the surface of a neutral atom of hydrogen were greater than the kinetic energy of a corpuscle moving with the velocity of 2 x 10$8$ cm./sec. This kinetic energy is equivalent to the fall of the atomic charge through 11 volts; hence we see that it must require an ionizing potential of more than 11 volts to liberate the corpuscle from a negatively electrified atom of hydrogen. The same considerations show that to liberate the corpuscle from a negatively electrified atom of carbon would require at least 9 volts, while for oxygen the corresponding ratio would be 7 volts. It must be remembered that these are merely inferior limits: the actual values may be much larger.

As we stated before, it is only some neutral atoms which are able to capture corpuscles when moving with the speed of the positive rays and only very few molecules are able to do so. The nature of the attraction between a neutral atom and an electric charge must, if we regard the atom as made up of corpuscles and positive charges, depend on the freedom with which the corpuscles can move under the field exerted by the charge: if they can move readily the force may be considerable. If on the other hand they are rigidly connected with the atom the force will be very small. A very simple experiment will illustrate this point. Suppose we have a considerable number of small compass needles with agate caps placed on a disk which is suspended from a long string. If we mount the com- pass needles on needle points fixed to the disk, so that they can turn freely, and then hold a magnet near the disk, the disk of the attraction between a neutral atom and a corpuscle.?

will be strongly attracted towards the magnet. If we take the needles off the needle points and lay them on the disk, the friction will prevent their rotation relative to the disk. If we now put the magnet In the same position as It was before It will be found that the attraction has been very much diminished.

Thus we should expect the attraction between a neutral atom and a corpuscle to be very much Increased by the presence In the atom of corpuscles which can move freely relatively to the atom. If these freely moving corpuscles are which are near the surface and which give rise to the forces which bind the atoms In the molecule together, we can readily understand why a neutral molecule should not attract a corpuscle as vigorously as a neutral atom. For when two atoms in a molecule are held together by the forces which they exert on each, the corpuscles in each atom will take up definite positions in their atoms, and will resist any displacement. Their mobility will thus be diminished and they will therefore not exert much attraction on a charge of electricity outside them. We Infer that those atoms which like helium do not occur among the positive rays with a negative charge have very few free corpuscles. It is.remarkable that so far as we know the atoms of the monatomic gases never occur with a negative charge in these experiments; this Is consistent with the preceding theory, for the attraction between two atoms depends on the presence of these mobile corpuscles, and If these are few or sluggish the force may be Insufficient to keep two atoms together. It would, as we have seen, require a strong attraction between an atom and a corpuscle to enable the atom to capture the corpuscle In experiments like these we are considering, when the atom moves past the corpuscle with a very great velocity. Franck$1$ has shown that even when this velocity Is comparatively small the atoms of a monatomic gas like argon cannot capture a negatively electrified corpuscle.

I have found two cases In which the molecule has occurred with a negative charge, the first of these is carbon. When the discharge tube contains such gases as CH$4$, CO$2$, CO where there are no bonds between two carbon atoms in the molecule, we find negatively charged carbon atoms but no negatively charged molecules. When, however, we use compounds such as acetylene ethylene, or ethane , where, according to the usual interpretation of the constitution of these bodies there are bonds between two carbon atoms In the molecule, then we find molecules as well as atoms of carbon with the negative charge. This is a very Interesting result as It shows (1) that there are strong forces between two carbon atoms In a molecule of the compound, forces strong enough to keep them united when the compound molecule is split up ; (2) that the corpuscles in the constituent atoms of the carbon molecule have considerable mobility, i.e. that the pair of carbon atoms Is not saturated in the sense that a pair of atoms of hydrogen or nitrogen are saturated when they form a molecule. These conclusions are in good agreement with chemical theory. With benzene vapour in the discharge tube I have found, In addition to negatively charged atoms or molecules, negatively charged triplets containing three carbon atoms. I have sometimes thought that in this case I could see indications of a line corresponding to four carbon atoms with a negative charge, but the line has always been so exceedingly faint that I cannot be sufficiently certain of the accuracy of the measurement to be quite sure that It was due to C$4$.

The other case in which I have observed a negatively charged molecule is that of oxygen; the negatively charged oxygen atom produces what in many cases is the strongest line on the negative side: the negatively charged oxygen molecule is only met with in exceptional cases. The causes which determine its appearance have not yet been made out: it probably depends on the presence in the tube of some special type of carbon compound. It does not seem to occur in very carefully purified oxygen, I have found it most frequently in oxygen containing a little hydrogen.

$1$ Franck, "Verhand. d. D. Phys. Ges." 12, 613, 1910.


text:
User talk:65.175.242.91

Welcome

Hello, and welcome to Wikisource! Thank you for your contributions. I hope you like the place and decide to stay. Here are a few good links for newcomers:

 * Help pages
 * Style Guide
 * Inclusion policy

You are welcome to continue editing articles without logging in, but you may wish to  [ create an account] . Doing so is free, requires no personal information, and provides several benefits. For one thing, if you continue to edit without a username, your IP address is used to publicly identify you.

In any case, I hope you enjoy donating your time to grow the Wikisource library that is free for everyone to use! In discussions, please sign your comments using four tildes ( ~ ); this will automatically produce your IP address (or username if you're logged in) and the date. If you need help, ask me on my talk page, or ask your question here (click  [ edit] ) and place  before your question. Again, welcome! -- George Orwell III (talk) 19:26, 1 January 2011 (UTC) A contribution from this account or ip address, which appeared to be a test edit, was removed or reverted. Please use the sandbox for this purpose. The community portal has information on contributing to this library.

title: User talk:65.175.242.91


text:
Rays of Positive Electricity and Their Application to Chemical Analyses/Atoms Carrying Two or More Positive Charges

Though the heads of most of the parabolic arcs are situated in the same vertical line, in many cases some of the parabolas, especially those corresponding to the atoms of oxygen and carbon, are prolonged towards the vertical axis. The prolongations do not reach right up to this axis but in many cases, as in the line a in Fig. 26, Plate II., which is due to the atom of oxygen, stop after going half-way. These prolongations of the parabolas are also parabolic and are continuations of the primary parabola. They are therefore due to particles which, when they are in the deflecting fields, have the same value of e/m as the particles which produce the primary parabolas. The fact that the smallest horizontal deflection of the prolongation is just half that of the corresponding deflection of the primary shows (see p. 12) that the swiftest of the particles in the prolongation has twice the kinetic energy of the swiftest,in the primary. Thus these particles when in the electric field in the discharge tube acquire twice the kinetic energy of the normal particle; they must therefore when in the discharge tube have had twice the normal charge. They must, after passing through the cathode and before getting into the deflecting fields, have had their charge reduced to the normal value. For as we have seen, the value of e/m in these fields Is normal, hence if they have retained the double charge they must have double the mass. If, however, they had retained the double charge the electrostatic deflection would have been normal: for though the kinetic energy Is doubled, which halves the deflection for normal charge, the charge and therefore the electrostatic deflection for given kinetic energy Is doubled too, and hence the result would be the normal deflection, while the actual deflection Is only onehalf of this. We conclude, therefore, that the prolongation Is due to particles which had a double charge when in the discharge tube, but which have lost one of these charges after passing through the cathode.

It is a strong confirmation of this view that when we find these prolongations we generally find on the same plate lines having a value of e/m twice that of the prolongation and beginning In the normal place; these are due to particles which have retained their double charge after passing through the cathode. And conversely when we find these lines corresponding to the double value of e/m we find a tail or prolongation to the line corresponding to the normal value. This would not necessarily be true at pressures so low that the particles did not make any collisions after passing through the face of the cathode, but I have not been able to reduce the pressure to this point.

The prolongations of the parabolas in some cases extend much more than half-way to the vertical axis; this is especially the case with the parabola produced by the positively charged atom of mercury. Fig. 27, Plate II, shows that even when the electric and magnetic fields are strong enough to produce several millimetres deflection in the heads of the parabolas corresponding to the other elements, the head of the mercury parabola is so little deflected that at first sight it seems to coincide with the origin. When exceedingly large electric fields are used it can be seen, however, that the head of the mercury parabola is distinctly displaced, and on measuring the amount of the de-flection it is found to be one-eighth of the normal displacement of the heads of the parabolas corresponding to the other elements.

This, as we have seen, implies that the particles which produce the head of the parabola corresponding to the atom of mercury must have eight times the maximum amount of energy possessed by the normal atom; if the theory given above is true, this means that some of the mercury atoms had, before passing through the cathode, eight times the normal charge, i.e. had lost eight corpuscles. Eight corpuscles is a very large number for an atom to lose, so that if in this case we can obtain independent evidence of such a loss it will be a strong confirmation of the theory.

A study of plates taken with large electrostatic deflections has revealed the existence of seven parabolas due to mercury, corresponding to the mercury atom with 1, 2, 3, 4, 5, 6, 7 charges respectively. The parabola corresponding to eight charges has not been detected, but as the parabolas in general get fainter for each additional charge, it is probably on the plate although not intense enough to be visible. Fig. 28, Plate II., taken from a photograph when the gas in the tube was the residual gas left after exhaustion by the Gaede pump, shows these lines very well. The measurements of m/e for the parabolas on this plate give the following value m/e is taken as unity for the atom of hydrogen) : —

 * {| style="text-align: left;width: 500px;"

! m/e !
 * 200
 * 200/1
 * 102
 * 200/2
 * 50.4
 * 200/4
 * 44
 * this Is not a mercury line but is due to CO$2$
 * 39.8
 * 200/5
 * 33.7
 * 200/6
 * 28.6
 * 200/7
 * }
 * 33.7
 * 200/6
 * 28.6
 * 200/7
 * }
 * 200/7
 * }

It will be noticed that the heads of the parabolas corres- ponding to 1, 2, 3. . . charges respectively lie on a straight line passing through the origin. This shows (p. 12) that the particles which produce these heads are all moving with the same velocity, and therefore, since each particle is an atom of mercury, that the kinetic energy of the particles at the heads of the parabolas is constant. This is in agreement with the theory, for the heads of all the parabolas are due to particles which before passing through the cathode had lost eight corpuscles. The particles at the head of the parabola corresponding to one charge (m/e= 200) has regained seven of these after passing through the cathode; the one at the head of the parabola corresponding to two charges (m/e= 100) has regained six and so on ; as the charge on these particles when they were in the discharge tube was eight units in each case, they would naturally acquire the same amount of kinetic energy before passing through the cathode.

The question now arises as to how the mercury atom acquires these very various charges. Can an atom of mercury when ionized lose any number of corpuscles from one to eight, or does it always lose a definite number? Take for example a mercury atom with five positive charges: has it got into this condition by losing five charges when it was ionized, or did it originally lose the maximum number eight and regain three subsequently ? The photographs suggest, I think that the second supposition is the correct one, and that in the discharge tube there are two and only two kinds of ionization. By one of these kinds the mercury atom loses one corpuscle, by the other eight The evidence for this is as follows: let us suppose for a moment that atoms with any charges from one to eight were produced by the ionlzation of the atoms of mercury in the discharge tube, and consider what effect this would have on the parabolas corresponding to the mercury  atom with one charge. This would be due to atoms of the following kinds :—

Atoms which had lost
 * (1) 8 corpuscles In the discharge tube and regained 7 subsequently.
 * (2) 6 corpuscles In the discharge tube and regained 6 subsequently.
 * (3) 6 corpuscles In the discharge tube and regained 5 subsequently.

and so on: the last member of the series being atoms which had lost one corpuscle on ionization and had not regained it.

The parabola seen on the plate would be due to the superposition of the eight parabolas due to these different types of atoms. The head of each of these parabolas would be separated from the head of any of the others: if d were the horizontal de- flection of the one due to the atom which had only lost one corpuscle in the discharge tube, d/2, d/3, d/4 d/5, d/6 d/7 d/8 would be the horizontal deflection of the heads of the parabolas due to the atoms which had lost 2, 3, 4, 5, 6, 7, 8 respectively. Thus the resultant parabola would for the part which had a horizontal deflection between d/8 and d/7 consist only of the parabola due to atoms of class (1); the part when the horizontal deflection was between d/7 and d/6 would consist of two parabolas due to the atoms of classes (1) and (2) ; the part with the horizontal deflection between d/6 and d/5 would be made up of the three parabolas corresponding to the atoms belonging to classes (1), (2), (3), and so on. Thus at the distance d/7, d/6, d/5, d/4, d/3, d/2 and d/1 we should expect an abrupt increase in the brightness of the curve, for at each of these places a new parabola is added to the old ones ; the intensity of the curve would thus not vary continuously but would have a beaded appearance. The abrupt increase in intensity at the distance d is very marked in the parabola; it is however, the only one to be detected. The intensity of this parabola is very great and it might be thought that the charges in the intensity might escape detection owing to the breadth of the curve. We may, however, apply the same reasoning to the parabolas which correspond to mercury atoms with three or four charges which are fine and well defined. The intensity of these curves is, however, perfectly continuous and there are no signs of the abrupt variations which ought to occur if the mercury atoms in the discharge tube had charges intermediate between one and eight. This result suggests that the ionization is mainly at any rate of two types, in the one type an atom of mercury loses a single corpuscle, In the other it loses eight There would thus seem to be two different agents producing ionization in the discharge tube. This is in accordance with another effect shown by many of the plates; on these plates there are well marked differences between the appearance of the lines due to charged atoms and those due to charged molecules. These differences are of various kinds: for example on the plate reproduced in Fig. 29, Plate III, the line α due to the hydrogen atom is of uniform intensity throughout, while β the one due to the hydrogen molecule, is very faint at the head but intense elsewhere; in others the line due to the atom is uniform while that due to the molecule has a beaded appearance. An example of this is shown in Fig. 30, Plate III. Perhaps the most important point shown by these plates is that when in a mixture of hydrogen and oxygen there are such differences between the lines due to the atoms and molecules of hydrogen, there are similar differences between the lines due to the atoms and molecules of oxygen. A similar result is obtained when the discharge passes through mixtures of nitrogen and hydrogen. It is interesting to observe that if we have along with these gases monatomic gases such as helium or mercury vapour, the lines corresponding to the atoms of these gases show the characteristics of both the and molecular lines of the diatomic gases, suggesting that some of the atoms of the monatomic gas have been Ionized by the process as the atoms of the diatomic gas, and others by the process which produced the charged molecules of this gas. There are in the discharge tube at least two kinds of ionizing agents, and it Is not unlikely that they produce different types of ionization. These agents are (1) rapidly moving cathode particles moving away from the cathode, and (2) positively charged atoms and molecules moving towards it; either of these agents can, as Is well known, produce ionization by collision. We should expect that the cathode particles, since they penetrate into the atom and come Into contact with the corpuscles Individually—the collision in favourable cases resulting In the detachment of a corpuscle—would give rise to singly charged systems, and If they struck a molecule might detach a corpuscle from one of the atoms without separating one atom from another, producing in this way a positively charged molecule. The collisions between the positively charged atoms and the atoms and molecules of the gas through which they are passing, might be expected to make the atom or molecule struck move off at a considerable velocity, which at first would not be shared by the corpuscles Inside the atom. The tendency of the corpuscles to leave the atom depends only upon the relative velocity of the atom and the corpuscles inside it, so that the ionizing effect produced by such a collision Is the same as If the atom were at rest, and all the corpuscles moving with the velocity acquired by the atom in the collision. Thus If there were several corpuscles connected with about the same firmness to the atom, the result of the atom acquiring a high velocity in a collision might be the liberation of all the corpuscles and the production of a multiply charged atom. Such a collision, since it would give one atom in a molecule a great velocity relatively to the other, would tend to dissociate the molecule Into atoms and produce positively charged atoms rather than molecules.

The maximum number of charges carried by a multiply charged atom does not seem to be related to any chemical property of the atom such as Its valency, but to depend mainly on the atomic weight; thus mercury, the most massive atom on which observations have been made, can have as many as eight charges, crypton atomic weight (82) four or five, argon atomic weight (40) three, neon atomic weight (20) two, nitrogen atomic weight (14), and oxygen (16) two, perhaps in rare cases three, helium also occurs with two charges; the multiple charge has been found on the atoms of all the elements tested with the very suggestive exception of hydrogen: no hydrogen atom with more than one charge has ever been observed, though as the hydrogen lines occur practically on every plate more observations have been made on the hydrogen lines than on those of any other element.

When there are on the plates lines corresponding to atoms of the same element with one, two, three charges, then the larger the number of charges the fainter the line. Judging from the intensity of the lines we should conclude that the number of multiply charged atoms is only a small fraction of the number with one charge. The ratio of the number of atoms which have only one charge to that of those which have two or more charges is very variable and depends on conditions which are not yet fully understood. For example in the case of the carbon atom this ratio seems to depend to a very great extent on the type of gaseous carbon compound in the discharge tube. With some hydrocarbons the doubly charged carbon atoms are relatively much brighter than with others. Again, in the case of oxygen I have found that the purer the oxygen the fainter was the line due to the doubly charged oxygen atom in comparison with that due to the atom with only one charge. It would thus seem that atoms torn from chemical compounds were more likely to have a double charge than those obtained from a molecule of the element. Chemical combination can not, however, be the only means by which the atoms acquire multiple charges, for the atoms of the inert monatomic gases, neon, argon and crypton, are remarkable for the ease with which they acquire multiple charges.

I have not been able to find any case in which a molecule of either an elementary or compound gas carries a double charge. The line corresponding to the molecule of nitrogen appears on some plates to have a prolongation towards the vertical axis; this would imply a double charge on the nitrogen molecule. I am inclined to think that this prolongation is not really due to the nitrogen molecule, but to the atom of aluminium, as m/e for this atom is 27.5, and for the nitrogen molecule 28, the lines would be so close together that It would be difficult to differentiate them.

Charged atoms on the view we have been discussing are in general produced by the impact of other charged atoms or molecules, while charged molecules are produced by the impact of cathode rays. This must not be taken to imply that cathode rays never produce charged atoms; it is probable that they would do so if they hit one of those corpuscles in the molecule which help, by the forces they exert, to bind the two atoms in the molecule together. There is direct evidence that in some cases charged atoms are produced by cathode rays, for Fulcher ("Astrophysical Journal," 34, p. 388) has shown that the passage of cathode rays through a gas generates in some cases the line spectrum of the gas, and line spectra are regarded as arising from atoms and not from molecules.

But though cathode rays may produce some charged atoms they more frequently produce charged molecules, the chief source of the charged atoms being positive rays, i.e. rapidly moving charged molecules or atoms. The view that the charged atoms and molecules are produced by different agents helps us to understand the remarkable variations which occur in the relative intensities of the lines due to the atoms and molecules of the same element. To take the case of hydrogen, sometimes the line due to the atom is stronger than that due to the molecule, at others it is weaker. Examples of this are shown in Figs. 31 and 32, Plate III.

Very small variations in the conditions of the discharge are sufficient to produce wide variations in the relative intensities of the atomic and molecular lines. If, for example, the cathode is placed so that its face comes inside the neck of the discharge tube as in Fig. 33a, the atomic line of hydrogen is stronger than the molecular: it is weaker, however, when the face protrudes beyond the mouth of the neck as in Fig. 33b. When the cathode is in the position indicated by Fig. 33a the pressure, when the positive rays are at their best, is higher than when the cathode is placed as in Fig. 33b ; so that this result suggests that, as the pressure increases, the intensity of the lines due to the atoms as compared with that of those due to the molecules increases also.


text:
Rays of Positive Electricity and Their Application to Chemical Analyses/Methods for Measuring the Number of the Positively Electric Fied Particles

Though the photographic plate furnishes an excellent means of detecting the existence of positively charged particles of different kinds it Is not suitable for comparing the number of particles present in a bundle of positive rays. For though the intensity of the lines on the photograph will vary with the number of particles, this number will not be the only factor In the expression for the intensity. As an example, consider the lines due (1) to very light particles like the atoms of hydrogen, and (2) to very, heavy ones like the atoms of mercury. If these particles have acquired the same amount of energy in the electric field before entering the cathode, the hydrogen atoms will have a velocity about fourteen times that of the mercury ones: they might therefore be expected to penetrate further into the film on the plate and produce a greater photographic effect than the mercury ones. If this expectation Is realized, and we shall see that it is, It Is evident that the photographic effect cannot be taken as a measure of the number of positively electrified particles.

A method which does give metrical results is founded on the following principle. Suppose that we replace the photographic plate in the preceding method by a metal plate in which there is a movable parabolic slit, then when this slit is moved into such a position that it coincides with one of the parabolas on the photographic plate, positively electrified particles would pass through the slit; if these particles are caught and their total charge measured we shall have a measure of the number of positively electrified particles. Thus if the slit were gradually moved up the plate there would be no charge coming through it, unless it coincided in position with one of the parabolas. As one parabola after another was passed, positive electricity would come abruptly through the slit, and the amount of the charge would be a measure of the number of particles passing through the slit. If instead of moving the parabolic slit we keep the slit fixed and gradually increase the magnetic field used to deflect the particles, we shall in this way drive one parabola after another on to the slit, beginning with the parabola due to the hydrogen atom and ending with that due to the mercury one, and the charges passing the slits will be proportional to the number of particles.

The apparatus used to carry this idea into practice is represented in Fig. 34. After passing through the electric and magnetic fields the particles, instead of falling on a photographic plate, fall on the end of a closed cylindrical metal box B. In the end of this box nearest the cathode a parabolic slit about 1 mm. in width is cut, the vertex of the parabola being the point where the undeflected rays would strike the box, and the tangent at the vertex the line along which the particles would be deflected by the magnetic force alone. This slit Is the only entry into the box. Inside the box and immediately behind the slit there is an insulated long, narrow vessel placed so that every particle passing through the slit falls into this vessel. This vessel is connected with a Wilson tilted electroscope by which the charge it receives can be measured.

From the front face of the box a portion was cut away, and the opening closed by a willemite screen. The positive rays could be deflected on to this screen and the brightness of the fluorescence observed; in this way one can make sure that the tube is in the proper state for giving positive rays before attempting to make the measurements.

The impact on the face of the box of the rays which do not pass through the slit gives rise to the emission of slowly moving cathode rays; if precautions are not taken these diffuse through the slit, enter the Faraday cylinder, and confuse the measurements. This diffusion can be avoided by placing a small permanent magnet near the slit The force due to this is strong enough to deflect the more mobile cathode rays without producing any appreciable effect on the positively charged atoms. The pressure of the gas between this box and the cathode should be made as small as possible: the best way of reducing the pressure is to absorb the gas by means of charcoal cooled with liquid air. This method will not produce a good vacuum when the gas in the tube is helium; with hydrogen, too, the vacuum is not so good as for heavier gases, for then the pressure can by this means easily be reduced to 3/1000 of a millimeter.

The method of observing with this apparatus is as follows: The positive rays are deflected by a constant electric field of such a magnitude that the heads of the parabolas are in line with one end of the slit The magnetic field is then increased by small increments and the deflection of the Wilson electroscope in ten seconds measured, Unless a parabola comes on the slit there Is practically no deflection; as soon, however, as the magnetic force is such that a parabola comes on the slit, there Is a considerable deflection which disappears when the magnetic force Is increased so as to drive the parabola past the slit The appearance and disappearance of the deflection of the electroscope are surprisingly sharp, so that lines quite near each other can be detected and separated. An example of the results obtained by this method Is given in Fig. 35. The abscissae are the values of the magnetic force used to deflect the rays, and the ordinates the deflection of the Wilson electroscope in 10 seconds. The gas in the tube was carbon monoxide.

A comparison of this curve with a photograph of the discharge through the same gas shows many interesting features. On the photograph the strongest lines are those corresponding to the atom and molecules of hydrogen. The curve on the other hand shows that the number of hydrogen particles is only a small fraction of the number of CO particles. The extraordinary sensitiveness of the photographic plate for the hydrogen atom in comparison with that for atoms and molecules of other gases is shown in all the curves taken by this method. But great as is the discrepancy in the case of the photographic plate between the effects produced by hydrogen atoms and an equal number of heavier atoms, it is not nearly so great as It is for a willemite screen: such a screen may show the hydrogen, lines very brightly while the CO line is hardly visible, when measurements made with the electroscope In the way just described show that the number of particles of hydrogen is only a few per cent of the number of the CO particles.

It is difficult to get from the photographs any estimate of the relative amount of the different gases in the discharge tube when it contains a mixture of several gases; for example, if the tube is filled with a mixture of hydrogen and oxygen the relative quantities of these gases may be varied within wide limits without producing any very marked effect on the relative brightness of the hydrogen and oxygen lines in the photograph. This electroscope method is much more metrical as will be seen from Figs. 36 and 37, the first of which represents the curve when the gas in the tube was a mixture of one-third hydrogen and two-thirds oxygen, while in the second, the gas was one-third oxygen and two-thirds hydrogen,

The negatively charged hydrogen atoms seem to have the same preponderance in their effect on the photographic plate over other negative atoms as positive hydrogen atoms have over other positive atoms. Thus on all the plates the line corresponding to the negatively electrified hydrogen atoms is well marked, often being comparable with the negatively electrified oxygen atom. With the electroscopic method the negative hydrogen atom can only just be detected, while the negatively electrified oxygen atoms produce a large negative deflection. A curve showing the comparative  numbers of different kinds of negatively electrified atoms is shown in the curve

Fig. 38: the gas in the tube was phosgene, COCl$2$; the curve at the top of the figure represents the number of negatively electrified particles, the one at the bottom the positively electrified ones. It will be seen that the negative atoms detected by the electroscopic method were carbon, oxygen, and chlorine, and that the chlorine atoms were by far the most numerous. On the photographs taken with this gas the line due to negatively electrified hydrogen seemed comparable in intensity with that due to negative chlorine. An interesting point about the curve representing the distribution of positively electrified, atoms is the great variety of atoms and molecules in this case, thus we find atoms of carbon, oxygen, and chlorine, and the molecules CO, Cl$2$, CCl and COCl$2$. It will be noticed that only a small fraction of the current is carried by free carbon and oxygen atoms, showing that in phosgene the carbon and oxygen atoms are so firmly united that the greater part of them remain together even when the gas is dissociated.

Are the atoms in a molecule of a compound gas charged with electricity of opposite signs?

The study of the curves obtained by the electroscopic method throws some light on the electrical states of the two atoms in a diatomic molecule of an elementary or compound gas. If we regard the forces which keep the atoms together as electrical in their origin, the question naturally arises, are the two atoms in a molecule of hydrogen, for example, charged one with positive the other with negative electricity; or in a molecule of hydrochloric acid gas is the hydrogen atom positively charged, the chlorine negatively, or in a compound like ammonia NH$3$ does the nitrogen atom carry three negative charges and each of the hydrogen atoms one positive one ?

Let us consider the case of CO for which we have in Fig. 35 the curve which represents the relative numbers of the different kinds of positively charged atoms. If the carbon atom in the molecule were positively, the oxygen atom negatively electrified, then we should expect that if a molecule of CO were split into atoms by the impact of a rapidly moving positively electrified particle, there would be a tendency for the carbon atoms to have a positive charge and for the oxygen ones to have a negative, so that in the positive rays we should expect to find more carbon atoms than oxygen ones. The curve, Fig. 36, shows that the number of positively electrified carbon atoms exceeds that of the positively charged oxygen ones in the proportion of 11 to 7. These figures, however, underrate the number of oxygen atoms which came through the cathode, for some of them after passing through the cathode acquired a negative charge. The charges given to the electroscope show that the proportion between negatively and positively charged oxygen atoms was as 2 to 7, while the number of carbon atoms which were negatively charged was very small in comparison with that of the positively charged atoms. Taking the negative atoms into account as well as the positive we find that the proportion between the number of carbon and oxygen atoms passing through the cathode is as 11 to 9; the numbers are too nearly equal to allow us to suppose that in the molecule one of the atoms is positively, the other negatively charged

The curve for COCl$2$, Fig. 38, shows that the proportion of positively electrified chlorine atoms in the positive rays Is not very different from the proportion of chlorine atoms in the normal gas. If the atoms in the molecule COCl$2$ had individually carried electric charges we should have expected the atoms of the strongly electro-negative element chlorine to have carried a negative charge and to have been relatively deficient in the positive rays.

The view that each of the atoms in a molecule of a compound contains as much positive as negative electricity is supported by considerations drawn from other branches of physics. If the atoms in a molecule of a gas carried separate charges so that one kind of atom was positively, another negatively, charged, then if the gas were dissociated into these atoms the atoms would be charged and the dissociated gas would be a good conductor of electricity. Now there are several gases which are dissociated at low temperatures, nickel carbonyl, for example, is at 100° C split up into nickel and CO to a very large extent; If these atoms were charged the electrical conductivity of the gas might be expected to begin to show marked increase at a temperature of about 70° C. when the dissociation first becomes appreciable. The variation of the conductivity of nickel carbonyl with temperature is, however, as Prof. Smith has shown, quite normal, following the same laws as for an undissociated gas. L. Bloch,$1$ too, has shown that the dissociation of arseniuretted hydrogen which also takes place at low temperatures Is not accompanied by any Increase in electrical conductivity. He also showed that many chemical reactions between gases which go on at low tempera- tures such as the oxidation of nitrogen bioxidCj the action of chlorine on arsenic, the oxidation of ether vapour, have little or no effect on the conductivity.

Chemical action, unless accompanied by high temperature, has not been shown to give conductivity. The very vigorous combination of hydrogen and chlorine under sunlight seems to have absolutely no effect on the conductivity of the mixture, and this Is a strong reason for supposing that the atoms In the molecule are not charged.

It Is true that chemical actions vigorous enough to raise the gases to a very high temperature, such as, for example, the combination of hydrogen and oxygen In the oxy-hydrogen flame, the oxidation in a Bunsen flame, the burning of CO and so on, make the reacting gases good conductors of electricity. This conductivity seems, however, from the result of recent experiments, to be due to the high temperatures produced by the chemical action rather than to that action itself. The conductivity cannot be due to the molecule being dissociated into positively and negatively electrified atoms, for the determinations of the mobility of the negatively electrified particles in flames and gases at a very high temperature show that it is much larger than would be possible If these particles had masses comparable with that of even the lightest atom, These negatively electrified particles are corpuscles, not atoms, and the electrical conductivity of gases at high temperatures is due to the dissociation of the molecules and atoms Into positively charged molecules and atoms and negatively electrified corpuscles, and not to a dissociation such as occurs In solution when the electrified bodies are positively and negatively electrified atoms. The conductivity of hot gases seems to be an example of the emission of corpuscles from hot bodies, rather than to be directly connected with chemical combination. We know that when we raise solids such as metals, or still better, certain oxides to a high temperature they give out copious streams of corpuscles, and the conductivity of flames is better explained by supposing that gases possess this property to some extent than by attributing It to chemical action alone.

We are led by these results to regard the electrical forces which keep the atoms In a molecule together as due not to one atom being charged positively and the other negatively but to the displacement of the positive and negative electricity In each atom. Thus each atom acts like an electrical doublet, and attracts another atom In much the same way that two magnets attract each other,

On the Information Afforded by the Positive Rays as to the Constitution of a Gas, the Nature and Properties of the Molecules, and the Process of Ionization in a Discharge Tube
The results we have given above enable us to appreciate the importance of the positive rays In Investigating the con- stitution of gases and the properties of atoms and molecules.

In the first place they give a very direct and simple proof of the molecular constitution of gases. We have seen that the positive rays from a gas produce on the photographic plate a finite number of distinct and sharp parabolas. As each parabola corresponds to a different kind of charged particle this shows that there are only a limited number of different kinds of particles in the discharge tube. The sharpness of the lines shows that all the particles of the same chemical element have to a very high degree of accuracy the same mass. If the atoms of hydrogen, for example, differed appreciably in mass it would not be possible to get on the photographic plates parabolas as sharply defined as those which can be obtained when the tube through the cathode has a very fine bore. As far as we know at present the fineness of the line depends only on the bore of the tube. This would not be the case if there were any variation in the mass of the atoms, for then instead of a line we should get a band bounded on one side by the parabolas corresponding to the heaviest atom and on the other by that corresponding to the lightest.

Again we see that in some gases we have both atoms and molecules, in others only atoms. We can infer from the study of the curves produced by the positive rays that helium, for example, Is a monatomic gas, hydrogen and oxygen diatomic.

The rays show too that the atoms and molecules of the gases can be charged with electricity; all of them, as far as we know, with positive electricity, some of the atoms with negative as well. The circumstances are very unfavourable for a particle in the positive rays to get a negative charge, and we must not conclude that because an atom or molecule has not been observed to acquire a negative charge when in the positive rays it is incapable of doing so under more favourable circumstances.

We have seen too that the atoms of all the elements except hydrogen can acquire more than one unit of positive charge. The maximum number of such units seems to depend on the atomic weight of the atom, for mercury, the heaviest atom yet investigated, it Is as large as eight, for krypton four or five, for oxygen two, and so on. No undoubted case of a double charge on a molecule, whether of an element or a compound, has yet been observed in addition to the atoms and molecules of recognized elements these rays reveal the existence of and other combinations which are not known to exist permanently in the free state. Thus the positive rays from marsh gas CH$4$ show, in addition to the atom and molecule of hydrogen and the atom of carbon, the systems CH$1$, CH$2$, CH$3$, CH$4$. The radicle OH with a negative charge has also been found when water vapour was in the tube. We can detect by this method systems which have a very transitory existence, for they need only last long enough to travel from the discharge tube to the photographic plate, a journey which takes less than the millionth of a second.

Again (see p. 45) the rays show that with those compounds of carbon, which contain two or more carbon atoms united by bonds, two carbon atoms connected together are found in the positive rays, and since they are found with a negative as well as with a positive charge, the two carbon atoms united In this way cannot be saturated.

Let us now consider the evidence given by these rays on the question of ionlzatlon. We see from the inspection of the photographs that ionlzatlon in the discharge tube Is not exclusively nor even mainly the detachment of a corpuscle from a neutral molecule: this process would produce merely positively electrified molecules. The photographs show that the products of ionlzatlon are much more complex than this: for though we do find the positively electrified molecules of the gas through which the discharge Is passing, whether that gas be an elementary one like hydrogen or oxygen or a compound one like carbon monoxide or carbonic acid, these molecules are by no means the only kind of electrified particles in the rays, instead of there being only one kind of carrier for the positive electricity there are always considerable number of kinds : in an experiment with benzene vapour I counted seventeen distinct kinds of positive carriers. Indeed the splitting up of the gas by the discharge is sometlmes so complete that the photographs of the positive rays in different hydrocarbons may be almost identical. Thus, for example, the prominent lines for the vapours of methyl alcohoI, ether ((C$2$H$5$)$2$O) and dimethyl ether are identical, showing that they are due to the products of dissociation of the molecules of these substances and not to the molecules themselves; the lines due to these are very faint.

In the discharge tube then we have dissociation — the splitting up of molecules into atoms — as well as the detachment of a corpuscle from the molecules.

The agents present In the tube which are known by independent experiments to produce ionlzation are : —

 * 1. Cathode rays of varying velocities moving away from the cathode.
 * 2. Positive rays moving towards the cathode.
 * 3. Rays analogous to Röntgen rays, due to the impact of cathodic and positive rays against the molecules of the gas through which they are passing and also to the recombination of the ions of opposite signs.

The primary effect produced by the impact of a cathode ray with a molecule would be to detach a corpuscle from this molecule. If this corpuscle is one of the structural ones, i.e. one of those causing the attractions which hold the atoms In a molecule together, then the ionizatlon will be accompanied by the separation of the atoms In the molecule: we shall have dissociation as well as Ionlzation and the result will be a positively electrified atom. If, however, the corpuscle detached Is not a structural one the atoms In the molecule will not be separated and a positively electrified molecule will be the result. If this were the only method of ionization we should expect a considerable excess of positively electrified molecules over positively electrified atoms. It Is not, however, probable that any large fraction of the Ionization In the tube Is due to the direct action of the faster cathode rays. The amount of ionization due to such rays has been measured by Glasson$2$ who found, as Is Indicated by theory, that the number of ions produced by cathode ray per unit length of path varies inversely as the kinetic energy of the ray. For rays moving with a velocity of 4.7 x 10$9$ cm./sec. through air at a pressure of 1 mm. of mercury, he found that I.5 pairs of Ions were produced by each ray In travelling over 1 cm. In the experiments with positive rays the velocity of the faster cathode rays was considerably greater than 5 x 10$9$ cm./sec. This would reduce the ionization if the pressure In the gas remained the same, but the pressure in the gas in our experiments was less than .01 mm. of mercury, so that even If we neglect the diminution in ionization due to increase in velocity, a cathode ray would only produce 1.5 pairs of ions when it had travelled over a metre, a distance much greater than the length of the tube. We conclude that the ionization on the gas is not therefore in the main due to the fast cathode rays: It arises more probably from slow cathode rays and from positive particles. The positive ions from the negative glow when they get into the dark space soon acquire sufficient energy to ionize the gas, producing corpuscles and other positive ions. These secondary corpuscles will at first be moving slowly as they are in a region In the dark space where the electric field Is comparatively weak; they will be efficient ionizers as their velocity is small, and will produce other corpuscles by collision, these corpuscles will be in a still weaker electric field and therefore still more efficient ionizers, as it is not until the velocity of the cathode particles sinks below that due to a fall through about 200 volts, that the ionization due to these particles increases as the velocity decreases. Thus near the anode end of the dark space the number of slowly moving cathode rays will increase with very great rapidity, and the gas in this neighbourhood will be a mixture of molecules and comparatively slowly moving cathode rays. Though these rays are slow in comparison with those that have acquired the energy due to the fall in potential through the whole of the dark space, their kinetic energy is large enough to correspond to that due to the thermal agitation of a particle at a very high temperature. For example a corpuscle moving with a velocity 10$7$ cm./sec. has energy corresponding to that due to thermal agitation at 0° C.: one moving with a velocity of 10$8$ would have energy equal to that due to the thermal agitation at about 27000° C. A velocity of 10$8$ cm./sec. would be acquired by a corpuscle through a fall of potential of between 3 and 4 volts, so that the velocity of even the slowest cathode particles in the discharge tube will be considerably greater than this. If there were anything approaching to equipartition of energy between these corpuscles and the structural corpuscles of the atoms in a molecule, the latter would acquire so much energy that they might wander away from the places they ought to occupy if they are to keep the atoms in the molecule together. The result of this would be that the atoms would separate and some of the corpuscles inside them would have considerable kinetic energy. This energy might be sufficient to carry them outside the atoms and thus produce positively charged atoms: either of the atoms in a diatomic molecule might be positively electrified in this way. The seat of ionization of this kind would be at the end of the dark space next the negative glow, so the positively electrified' particles produced in this way would fall through the whole of the potential difference in the space, and would acquire the maximum amount of kinetic energy, they would therefore hit the photographic plate at the of the parabola corresponding to this kind of particle. Some of the photographs,, such as the one reproduced in Fig. 39, Plate IV,, have the heads of the parabolas very much more intense than the rest of the arc, indicating that in this case the majority of the particles have fallen through the maximum potential difference and therefore have been produced at the end of the dark space. There are many cases, however, where concentration does not occur, and where the arcs of the parabolas are very long, indicating that there is a very considerable range in the kinetic energy of the particles, the uniform intensity of the lines shows that the number of particles is fairly equally distributed over a considerable range of kinetic energy.

We might account for this variation in the kinetic energy in. the following ways:—

 * 1. By supposing that the charged particles all started from the same region, the end of the dark space, but before reaching the cathode got neutralized and so were only under the influence of the electric field for a fraction of the journey through the dark space.

 * 2. By supposing that the ionization which produced these particles occurred not merely at the end of the dark space but to some extent throughout the whole of this space.

 * 3. By supposing that the small amount of kinetic energy possessed by some of the particles is due to their colliding with the molecules of the gas whilst passing through the dark space and in this way losing some of their kinetic energy.

Let us begin with the first of these suggestions. We have seen (p. 32) that unless the pressure Is very low some of the particles, after they pass through the cathode, get neutralized and lose their charge for a time; in some cases they acquire it again by being ionized by collision with a corpuscle. If this process went on in front of the cathode as well as behind it we should get variations In the kinetic energy of the particles, as some of them would have passed a larger fraction of their time In the uncharged state than others. The corpuscles which produce neutralization and ionlzation must be those in the molecules of the gas through which the positive rays are passing and not the free corpuscles. For in the dark space in front of the cathode there is an intense electric field in which the free corpuscles are moving far faster than the particles so that the relative velocity of the particles and free corpuscles is much greater than that of the particles and the corpuscles in the molecules, and therefore the union of the positively electrified particles and the free negatively electrified corpuscles is not likely to occur; there may, however, be some neutralization owing to a positively charged particle uniting with a corpuscle from an uncharged molecule, through which the positive particle is passing.

The third suggestion that the loss of kinetic energy is due to collisions between the particles and the molecules in the discharge tube through which they are moving is open to the objection that It would produce effects of the same general character on all the lines. Thus If all the particles started from the end of the dark space and the reduction In their velocity was due to these collisions we should expect all the lines to show a general resemblance in the way the intensity varied along the parabola. We find, however, on the same plate lines which are quite short, with all the intensity concentrated at the head, and others which are long and of equal intensity throughout The second suggestion, that positive particles are produced at different parts of the field by other positively electrified particles in rapid motion, seems to me to Indicate an effect which must undoubtedly be largely instrumental In increasing the length of the parabolas. The particles which are produced near the cathode will not, when they reach the cathode, have fallen through as great a potential difference as those produced farther away and will therefore have a smaller velocity.

$1$ "Annajes de Chimte et de Physique," XXII, pp. 370, 441; XXIII, p. 28.

$2$ Phil. Mag.," October, 1911.