taking place slowly in the non-radio-active elements; or, in other words, that all matter may be undergoing a slow process of change. The changes taking place in the radio-elements have been observed only in consequence of the expulsion with great velocity of the parts of the disintegrated atom. Some recent experiments described in Appendix A show that the α particle from radium ceases to ionize the gas when its velocity falls below about 10^9 cms. per second. It is thus seen that α particles may be projected with a great velocity, and yet fail to produce ionization in the gas. In such a case, it would be difficult to follow the changes by the electrical method, as the electrical effects would be very small in comparison with those produced by the known radio-active bodies.
260. Radiations from the products. We have seen that
the great majority of the radio-active products break up with the
expulsion of α particles, and that the β particle with its accompaniment
of the γ ray appears in most cases only in the last rapid
change. In the case of radium, for example, which has been most
closely investigated on account of its great activity, radium itself,
the emanation and radium A emit only α particles; radium B
emits no rays at all; while radium C emits all three kinds of rays.
It is difficult to settle with certainty whether the products thorium
X and actinium X emit β particles or not, but the β and γ rays
certainly appear in each case in the last rapid change in the
active deposit, and, in this respect, behave in a similar manner
to radium.
The very slow moving electrons which accompany the particles emitted from radium (section 93) are not taken into account, for they appear to be liberated as a result of the impact of α particles on matter, and are expelled with a speed insignificant compared with that of the β particles emitted from radium C.
The appearance of β and γ rays only in the last rapid changes of the radio-elements is very remarkable, and cannot be regarded as a mere coincidence. The final expulsion of a β particle results in the appearance of a product of great stability, or, in the case of radium, of a product (radium D) which has far more stability than the preceding one. It would appear that the initial changes are accompanied by the expulsion of an α particle, and that once
