310 RADIO-ACTIVE PROCESSES [CH. 
the radio-element. The term “conservation of radio-activity” is 
thus a convenient expression of the facts known at the present 
time. It is quite possible, however, that further experiments at 
very high or very low temperatures may show that the radio- 
activity does vary. For example, Dewar states that the heat 
emission of radium seems to be rather greater in liquid hydrogen 
than in liquid oxygen. An increase of heat emission would 
probably entail an increase of the radio-activity of the radium 
immersed in liquid hydrogen. Accurate experiments have not, 
however, yet been made on the radio-activity of radium at such 
low temperatures. 
Although no difference has been observed in the radio-activity 
of uranium over an interval of five years, 1t will be shown (sec- 
tion 203) that on theoretical grounds the radio-activity of a given 
quantity of a radio-element should decrease with the time. The 
change will, however, be so slow in uranium and thorium, that 
probably thousands if not millions of years must elapse before a 
measurable change would have taken place. In radium, however, 
the change takes place about one million times faster, so that a 
measurable alteration should be detected in the course of a few 
years!. The total radio-activity of a given quantity of matter left 
to itself should thus decrease, but 1t should be constant for a 
constant mass of the radio-element. It is only in this restricted 
sense that the principle can be employed. 
The conservation of radio-activity applies not only to the 
radiations taken as a whole, but also to each specific type of 
radiation. If the emanation is removed from a radium compound, 
the amount of 8 radiation of the radium at once commences to 
decrease, but this is compensated by the appearance of 8 rays 
in the radiations from the vessel in which the separated emanation 
is stored. At any time the sum total of the 8 radiations from the 
radium and the emanation vessel is always the same as that from 
the radium compound before the emanation was removed. 
1 Tt seems probable however that the radio-activity of radium, measured by the 
a rays, will increase rather than diminish for several hundred years after its 
separation. This is due to the fact that the increase of the activity due to the last 
slow change of radium (about half changed in 200 years) will probably more than 
compensate for the change in the radium itself. Ultimately, however, the radio- 
activity of the radium must decrease with time. 
