the β particle is expelled, the components of the residual atom fall into an arrangement of fairly stable equilibrium, where the rate of transformation is very slow. It thus appears probable that the β particle, which is finally expelled, may be regarded as the active agent in promoting the disintegration of the radio-atom through the successive stages. A discussion of this question will be given with more advantage later (section 270), when the general question of the stability of the atom is under consideration.
It is significant that the change in which the three types of rays appear is far more violent in character than the preceding changes. Not only are the α particles expelled with greater velocity than in any other change, but the β particles are projected with a velocity very closely approaching that of light.
There is always a possibility that, in such a violent explosion in the atom, not only may the α and β particles be expelled, but the atom itself may be disrupted into several fragments. If the greater proportion of the matter resulting from the disintegration is of one kind, it would be difficult to detect the presence of a small quantity of rapidly changing matter from observations of the rate of decay; but, if the products have distinctive electro-chemical behaviour, a partial separation should, in some cases, be effected by electrolysis. It has already been pointed out that the results of Pegram and von Lerch (section 207) on the electrolysis of thorium solutions may be explained on the supposition that thorium A and B have distinctive electro-chemical behaviour. Pegram, however, in addition observed the presence of a product which decayed to half value in six minutes. This active product was obtained by electrolysing a solution of pure thorium salt, to which a small quantity of copper nitrate had been added. The copper deposit was slightly active and lost half of its activity in about six minutes.
The presence of such radio-active products, which do not come under the main scheme of changes, indicates that, at some stage of the disintegration, more than one substance results. In the violent disintegration which occurs in radium C and thorium B, such a result is to be expected, for it is not improbable that there are several arrangements whereby the constituents of the atom
