ea) 
304 RADIO-ACTIVE PROCESSES [CH. 
the radium is correspondingly diminished, but the activity of 
the radium compound is spontaneously recovered owing to the 
production of fresh emanation. A state of radio-active equi- 
librium is reached, when the rate of production of fresh emanation 
balances the rate of change in the emanation stored up im the 
compound. The differences observed in the rate of recovery of 
radium under different conditions were probably due to variations 
in the rate of escape of the emanation. 
193. Non-separable activity. It has been shown that, for 
all three radio-elements, uranium, thorium, and radium, there is 
a non-separable activity consisting entirely of a rays. In the 
case of uranium the activity 1s the same as the activity of the 
uranium, measured by the a rays, before the product Ur X, which 
gives rise only to B rays, is removed. In the case of thorium 
and radium, where the active products produced give out @ rays, 
the non-separable activity is about 25 per cent. of the maximum 
activity measured by the a@ rays. 
The existence of a non-separable activity follows from the 
point of view of regarding radio-active processes which has been 
advanced in sections 87 and 127. The three radio-elements 
are supposed to be undergoing atomic disintegration, which is 
accompanied by the expulsion of a rays. If the number of atoms 
which break up per second is almost infinitesimal compared with 
the total number present, the same number, on an average, will 
break up per second. The number of @ particles expelled per 
second will thus be a constant for each radio-element. There 
will thus always be a non-separable activity of the radio-elements, 
which is an inherent property of the elements and cannot be 
removed from them by any chemical or physical process. 
194. Radiations from the active products. Most of the 
changes occurring in the different radio-active products are accom- 
panied by the emission of a rays alone. The 8 and y rays appear 
only in the final stages of the radio-active changes. 
It has been shown that the non-separable activity of the three 
radio-elements consists entirely of arays. The two emanations give 
rise only to a rays (section 147). It also seems probable that the 
product Th X, if the emanation X which it produces is completely 
