582 Proi. Rutherford and Mr. fSoddy 



for the phenomenon generally. The radioactive constant X 

 has been investigated under very widely varied conditions of 

 temperature, and under the influence of the most powerful 

 chemical and physical agencies, and no alteration of its value 

 has been observed. The law forms in fact the mathematical 

 expression of a general principle to which we have been led 

 as the result of our investigations as a whole. Radioactivity, 

 according to present knowledge, must be regarded as the 

 result of a process which lies wholly outside the sphere of 

 known controllable forces, and cannot be created, altered, or 

 destroyed. Like gravitation, it is proportional only to the 

 quantity of matter involved, and in this restricted sense it is 

 therefore true to speak of the principle as the conservation 

 of radioactivity*. Radioactivity differs of course from gravi- 

 tation in being a special and not necessarily a universal 

 property of matter, which is possessed by different kinds in 

 widely different degree. In the processes of radioactivity 

 these different kinds change into one another and into 

 inactive matter, producing corresponding changes in the 

 radioactivity. Thus the decay of radioactivity is to be ascribed 

 to the disappearance of the active matter, and the recovery 

 of radioactivity to its production. When the two processes 

 balance — a condition very nearly fulfilled in the case of the 

 radio-elements in a closed space — the activity remains con- 

 stant. But here the apparent constancy is merely the 

 expression of the slow rate of change of the radio- element 

 itself. Over sufficiently long periods its radioactivity must 

 also decay according to the law of radioactive change, for 

 otherwise it would be necessary to look upon radioactive 

 change as involving the creation of matter. In the universe 

 therefore the total radioactivity must, according to our 

 present knowledge, be growing less and tending to disappear. 



* Apart from the considerations that follow, this nomenclature is a 

 convenient expression of the observed facts that the total radioactivity 

 (measured by the radiations peculiar to the radio-elements ) is for any 

 given mass of radio-element a constant under all conditions investigated. 

 The radioactive equilibrium may be disturbed and the activity distributed 

 among one or more active products capable of separation from the 

 original element. But the sum total throughout these operations is at 

 all times the same. 



For practical purposes the expression " conservation," applied to the 

 radioactivity of the three radio-elements, is justified by the extremely 

 minute proportion that can change in any interval over which it is 

 possible to extend actual observations. But rigidly the term " conser- 

 vation " applies only with reference to the radioactivity of any definite 

 quantity of radioactive matter, whereas in nature this quantity must be 

 changing spontaneously and continually growing less. To avoid possible 

 misunderstanding, therefore, it is necessary to use the expression only in 

 this restricted sense. 



