Curie and Debierne21 observed that when a vessel containing an emanating radium compound was kept pumped down to a low pressure, the amount of excited activity produced on the vessel was much reduced. In this case the emanation given off by the radium was removed by the pump with the other gases continuously evolved from the radium compound. On account of the very slow decay of activity of the emanation, the amount of excited activity produced on the walls of the vessel, in the passage of the emanation through it, was only a minute fraction of the amount produced when none of the emanation given off was allowed to escape.
190. Transmission of excited activity. The characteristic property of excited radio-activity is that it can be confined to the cathode in a strong electric field. Since the activity is due to a deposit of radio-active matter on the electrified surface, the matter must be transported by positively charged carriers. The experiments of Fehrle22 showed that the carriers of excited activity travel along the lines of force in an electric field. For example, when a small negatively charged metal plate was placed in the centre of a metal vessel containing an emanating thorium compound, more excited activity was produced on the sides and corners of the plate than at the central part.
A difficulty however arises in connection with the positive charge of the carrier. According to the view developed in section 136 and later in chapters X and XI, the active matter which is deposited on bodies and gives rise to excited activity, is itself derived from the emanation. The emanations of thorium and radium emit only α rays, i.e. positively charged particles. After the expulsion of an α particle, the residue, which is supposed to constitute the primary matter of the active deposit, should retain a negative charge, and be carried to the anode in an electric field. The exact opposite however is observed to be the case. The experimental evidence does not support the view that the positively charged α particles, expelled from the emanation, are directly responsible for the phenomena of excited activity; for no excited activity is produced in a body exposed to the α rays of the emanation, provided the emanation itself does not come in contact with it.
There has been a tendency to attach undue importance to this apparent discrepancy between theory and experiment. The difficulty is not so much to offer a probable explanation of the results as to select from a number of possible causes. While there can be little doubt that the main factor in the disintegration of the atom consists in the expulsion of an α particle carrying a positive charge, a complicated series of processes probably occurs before the residue of the atom is carried to the negative electrode. The experimental evidence suggests that one or more negative electrons of slow velocity escape from the atom at the same time as the particle. This is borne out by the recent discovery that the particle expelled from radium, freed from the ordinary β rays, and also from polonium, is accompanied by a number of slowly moving and consequently easily absorbed electrons. If two negative electrons escaped at the same time as the α particle, the residue would be left with a positive charge and would be carried to the negative electrode. There is also another experimental point which is of importance in this connection. In the absence of an electric field, the carriers remain in the gas for a considerable time and undergo their transformation in situ. There is also some evidence (section 227) that, even in an electric field, the carriers of the active deposit are not swept to the electrode immediately after the break up of the emanation, but remain some time in the gas before they gain a positive charge. It must be remembered that the atoms of the active deposit do not exist as a gas and by the process of diffusion would tend to collect together to form aggregates. These aggregates would act as small metallic particles, and, if they were electro-positive in regard to the gas, would gain a positive charge from the gas.