Page:Radio-activity.djvu/340

 '''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 Fehrle 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 and, 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