20 SODDY, Evolution of Matter by Radio-active Elements.. 



nature are the distinctive criteria employed. In the case 

 of the metabolons, physical and chemical nature can 

 sometimes be fairly completely studied. For example,, 

 the emanations have been found to possess that curious 

 power of resisting absorption by chemical reagents which 

 associates them in nature with the inert gases of the 

 argon family. They might be identified, moreover, by 

 their temperature of volatilisation and condensation, or 

 by their rates of diffusion. But these are somewhat 

 special cases. The most valuable criterion which is 

 universally applicable is the rate of change, and since the 

 universal law is that a definite fraction changes in the 

 unit of time, the rate of change is expressed by X, ' the 

 radio-active constant' which represents the proportionate 

 fraction changing per second. Thus the emanations of 

 radium and thorium are very similar in their general 

 characteristics, but are widely different in their rates of 

 change. X for the case of the thorium emanation is about 

 1/87, while for the radium emanation it is 6,000 times 

 smaller. The radio-active constant is determined by the 

 rate of decay of the activity of the transition-form. The 

 number of a rays expelled per second, i.e., the radio- 

 activity, is a measure of the number of metabolons 

 changing per second, and therefore, since a fixed fraction 

 changes per second, of the number remaining unchanged. 

 The activity decays in a geometrical progression with the 

 lapse of time, and the quantity diminishes accordingly. 

 If I„ is the original activity, and I^ the activity after time /,. 

 the value of X is given by the equation — 



\t -u I, 



:p = (? or lege y = -\t. 



■1-0 -'■0 



Thus the value of -r = -, i-^-> the quantity is diminished 

 to half the initial, in about one minute in the case of the 



