1 8 SODDY, Evolution of Matter by Radio-active Elements. 



nature of the bodies, and certain of the ordinary methods 

 of chemical analysis prove sufficient for the task. The 

 emanations and their subsequent products of change have 

 been the most studied, owing to the former being gaseous, 

 and so continually separating themselves by diffusion 

 (emanation) from the parent-elements producing them. 

 The fact that the emanations condense at the temperature 

 of liquid air affords the means whereby they can be freed 

 entirely frpm other gaseous matter present, and as we shall 

 consider later, has resulted, in the case of the emanation of 

 radium, in some important additions to our knowledge. 



All the transition-forms have the same general char- 

 acteristics. We have seen that they can only attain 

 infinitesimal quantity, and are known by their rapid rate 

 of change. It follows in consequence that their energy 

 is quickly dissipated ; the radio-activity continually decays 

 to zero in a geometrical progression with the time ; and 

 the rate of decay furnishes a measure of the rate of 

 change, and is a constant characteristic for each type of 

 matter concerned. Under ordinary circumstances the 

 radiations from a radio-element which has attained the 

 condition of radio-active equilibrium are contributed for 

 the most part by the various transition-forms which result 

 from its disintegration, and only a small part is derived 

 from the initial disintegration of the element itself But 

 this part cannot be separated by chemical means, because 

 nothing can stop a certain fraction of the total number of 

 atoms of the parent-element disintegrating per second. 

 This is the "non -separable radio-activity." Since the 

 fraction of the parent-element disintegrating is infini- 

 tesimal, the total amount does not perceptibly diminish. 

 The non-separable activity is therefore sensibly constant 

 even over long periods of time. 



The operation of separating the transition-forms from 



