306 RADIO-ACTIVE PROCESSES (CH: 
y rays appear at the same time as the @ rays, and in an amount 
proportional to them. 
The character of the radiations from each of the radio-active 
products must always be taken into consideration in the interpre- 
tation of results obtained by different methods of measurement. 
For example, a radium compound, which has been heated in an 
open glass tube, after a few hours practically loses its power of 
exciting fluorescence on a screen of platino-cyanide of barium 
placed near it. This is due to the fact already mentioned, that 
the 8 and y rays practically disappear from radium for several 
hours after the emanation is removed. For the same reason the 
radium, with a screen placed over it of sufficient thickness to 
absorb all the a rays, would appear, when measured by the electric 
method, to be almost inactive. 
Since the a rays are photographically very inactive compared 
with the 8 rays, the non-separable activity of uranium, thorium, 
and radium, although producing marked ionization, would be 
almost inappreciable if tested by the photographic method. It has 
been stated by some observers that uranium and thorium have been 
obtained which showed no trace of activity. On examination of the 
results, however, it is found that the methods employed were not 
suitable to definitely settle the question. It is true that, by certain 
chemical processes, uranium and thorium can be obtained tempo- 
rarily inactive, when tested by the photographic method, or by the 
electric method if the compound is covered by a screen of sufficient 
thickness to absorb all the arays. If however the activity is tested . 
electrically with unscreened active matter, there is always found to be 
a residual activity. In the course of time, the uranium and thorium 
compounds spontaneously regain the whole of their lost activity. 
195. Division of the activity amongst the products. 
It has been shown in section 190 that the activity of thorium 
hydroxide, after the removal of Th X, falls to 46 per cent. of its 
original value. When the Th X is removed from the thorium at 
short intervals, in order to allow the activity of the emanation X 
left. behind to decay, there is a residual activity of 25 per cent. of 
the maximum. About 21 per cent. of the total activity is thus 
due to the emanation X. 
