x] RADIO-ACTIVE PROCESSES 305 
separated from it, would give rise only to @ rays. On the other 
hand, the activity of Ur X consists only of 8 rays. The changes 
in the matter emanation X of thorium and radium give rise to 
both a and @ rays. It has been shown (section 176) that the 
emanation X of thorium goes through two changes, the first of 
which does not give rise to rays at all, and the second of which 
gives rise to aand 8 rays. The first change occurring in emana- 
tion X of radium gives rise to a rays but not to 8 rays; the 
second change probably does not give rise to rays at all, while 
the third change again includes all three kinds of rays. The 
absence of 9 rays in the first change, taking place in the emana- 
tion X of radium, can readily be shown by exposing a negatively 
charged wire in the presence of the radium emanation for a few 
minutes. The activity on removal, measured by the «@ rays, falls 
rapidly, as is shown in Fig. 51, p. 262; but the activity measured 
by the 8 rays alone is at first small, and increases for some time 
instead of diminishing. If a and £ rays had been both given out 
in the first change, it is to be expected that the amount of the @ 
radiation would initially decay at the same rate as the a radiation, 
but no such effect is observed. 
The 8 and probably also the y rays of the three radio-elements 
thus only appear in the last of the series of radio-active changes. 
It is remarkable that the last change, which is readily detected 
by the radio-active property, should in each of the three radio- 
elements be accompanied by the expulsion of a single electron 
with great velocity, and that all the other changes, with the ex- 
ception of two that probably give rise to no rays at all, should 
be accompanied by the expulsion of @ rays, 7.e. of material particles 
atomic in size. 
The polonium of Mme Curie and the radio-tellurium of 
Marckwald emit only a rays. Becquerel! states that he has 
detected some rays of a penetrating character from polonium 
by the photographic method. The writer has examined by the 
electric method the radiations from the active preparation of 
radio-tellurium, but was unable to detect any trace of y rays. 
The evidence so far obtained points to the conclusion that the 
1 C. R. pp. 136, 977. 1903. 
ihe Teh 20 
