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

by Joseph John Thomson

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 ½mv²/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 10⁸ 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 10⁸ 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. Franck¹ 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 CH₄, CO₂, 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 C₄.

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.

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