Manchester Memoirs, Vol. xlviii. (1904), No. 8. 15 



present day. The initial type of matter will be termed 

 the parent-element. For any ordinary quantity of parent- 

 element, considered over any ordinary period of time, the 

 actual quantity of the transition-forms of matter resulting 

 from the disintegration must be excessively minute. The 

 quantity of the transition-form produced, moreover, cannot 

 -accumulate beyond a certain limiting or equilibrium value, 

 which is attained when the amount changing per second 

 equals the amount produced per second. This condition 

 will be termed radio-active equilibrium. Since the rate of 

 production is slow, and the rate of change of the 

 transition-form is rapid, the equilibrium quantity must 

 always be practically infinitesimal, and quite below that 

 needed for detection by ordinary methods. But the rapid 

 rate of change of the transition-form and the expulsion of 

 a particles which accompanies the change, afford the means 

 whereby these transition-forms can readily be detected 

 and studied, although only present in so small quantity 

 that no other evidence of their existence is manifested. 



Looked at in the broadest and most general way the 

 sole criterion which determines whether a transition-form 

 ■can be detected experimentally or not is whether the 

 energy liberated from unit weight in unit time is 

 sufficiently great, and is manifested in a suitable manner 

 to be detected by our instruments. That the energy 

 liberated in the disruption of the atom is enormous is 

 sufficiently shown by the fact that a gram of radium can 

 liberate 100 calories per hour, year after year, without the 

 supply showing appreciable signs of diminution. We 

 have seen that a quantity of radium a million times less 

 than can be spectroscopically detected can be recognised 

 by its radio-activity, i.e., by its energy-emission. If this 

 ■energy-emission, instead of being spread over a term of 

 •centuries as it undoubtedly is in the case of radium, went 



