Rays of Positive Electricity and Their Application to Chemical Analyses/Disintegration of Metals Under the Action of Positive Rays
When positive rays strike against a metallic surface, the metal disintegrates and forms a deposit on the walls of the tube surrounding the metal. A well-known example of this is the "spluttering" of the cathode in a vacuum tube; another is observed when working with an apparatus like that in Effect at Very Low Pressures|Fig. 10; after long use the thin metal tube which through the cathode gets worn away at the end nearest the discharge tube, as if it had been struck by a sand blast. Sometimes several millimetres of the tube are destroyed in this way. An excellent account of the very numerous experiments which have been made on the spluttering of the cathode will be found in a report by Kohlschiitter ("Jahrbuch der Radioaktivitat," July, 1912).
The experiments of Holborn and Austin, Granqulsts and Kohlschütter indicate that with a constant current w the loss of weight in a given time may be represented by a formula of the type
where V is the cathode fall of potential, A the atomic weight of the metal, n a small positive integer, and a and S quantities which are much the same for all metals, or at any rate the metals can be divided into large classes and a, and S are the same for all the metals in one class. For a current of .6 milliamperes, Holborn and Austin found that for all the metals they tried S was 495 volts. We see that a formula, of this type implies that there is no spluttering unless the cathode fall of potential exceeds a definite value S and this seems to be verified by experience.
The experiments of Holborn and Austin, Kohlschiitter and others have shown that this expression for the loss of weight of the cathode fails when V exceeds a certain value, for hydrogen this value seems to be so low that the expression fails before the loss of weight becomes measurable.
The loss of weight of the six metals Al, Fe, Cu, Pt, Ag, Au have been measured by Kohlschütter and Müller ("Zeitschr. f. Elektroch.," 12, 365, 1906) and Kohlschiitter and Goldschmidt (ibid. 14, 221, 1908) in the gases H2, He, Na, O2 and Au, under as nearly as possible identical electrical conditions. They found that for all gases the amount of weight lost was in the order in which the metals are written above, gold always losing the greatest amount and aluminium the least. For the same meta! in different gases the loss of weight followed the order of the atomic weight of the gases, the loss in hydrogen being least and that in argon greatest. This may be connected with the fact that (see p. 47) elements of high atomic weight acquire multiple charges of electricity more easily than the lighter elements, and atoms with a multiple charge have more energy when they strike against the cathode than those which have only one charge. It is easy to understand in a general way why particles with the large amount of energy possessed by the positive rays when they strike against an atom in the cathode might communicate to it sufficient energy to enable it to escape from the cathode. A complete theory, however, is lacking, and one which would explain some of the more striking facts, such as why the value of S is so nearly the same for metals of very different physical and chemical properties, would probably throw a good deal of light upon some important properties of the atom.
Dechend and Hammer ("Zeitschr. f. Elektroch.," 17, 235) allowed the positive rays produced in sulphuretted hydrogen to pass through a perforated cathode and after deflection by magnetic and electric fields to fall upon a plate of polished silver, they could detect the parabolas on the plate, but while the parabolas due to hydrogen were so faint that they could only be detected as breath figures, those due to the heavier atoms, presumably sulphur, had so affected the plate that they could not be removed either by acid or rubbing. The greatest effect, however, was produced by the undeviated rays. In addition to the effects produced when the positive rays strike against a metal plate there is, as Schmidt has shown, a general oxidation over the surface when the metal is oxldizable and when the gas surrounding it contains oxygen. The passage of the positive rays through the oxygen produces atomic oxygen which is very active chemically and which attacks the plate. If, on the other hand, an oxidized plate is placed in hydrogen and exposed to the action of positive rays the oxide is reduced, the rays produce atomic hydrogen which acts as a strong reducing agent.
Some of the atoms constituting the positive rays seem to enter a metal against which they strike, and either combine with the metal or get absorbed by it. Helium, neon, and mercury vapour seem especially noticeable in this respect. If a cathode has once been used for any of these gases, positive rays corresponding to these elements will be found when the cathode is used with other gases, and it requires long continued discharge and repeated fillings with other gases before they are eliminated.
A very valuable Bibliography of Researches on Positive Rays has been published by Fulcher (Smithsonian Miscellaneous Collection, 5. p. 295, 1909).