MODERN THEORIES OF ELECTRICITY AND MATTER. 115 



ing j^rogressioii, characterized by a constant coefficient. This coeffi- 

 cient may be defined as the time required for the diminution of the 

 activity by half. These constants appear to be independent of the 

 conditions of experiment, are characteristic of the substance to which 

 they appertain, and seem to be capable of fixing an absolute scale of 

 time. Thus the emanation of radium diminishes by half in about 

 four days, while that of thorium diminishes by half in about one 

 minute, and that of actinium in about four seconds. 



I have already stated that the radioactivity is a general property of 

 matter. If the theory of radioactive transformation continues to 

 insj^ire a growing degree of confidence, it will result in an important 

 consequence for geology, and will lead to a careful study of the pro- 

 portions of the elements occurring in rocks, with a view to deduce 

 their relative ages. 



It is plain that the hypothesis of radioactive transformation is 

 well adapted to the present state of the science of radioactivity. 

 It Avas among those proposed by M. Curie and myself at the begin- 

 ning of our researches on radioactivity;* but it has received its 

 precise development by Rutherford and Soddy, to whom it is for 

 this reason generally attril:)uted. It seems to me, however, better 

 not to leave the domain of demonstrated facts, and not to lose sight 

 of other explanations of radioactivity which have been proposed. 

 The actual state of the science does not seem to me far enough 

 advanced to warrant a positive conclusion. 



In closing, the general importance of the phenomena of radio- 

 activity may be recalled. For physics the radioactive substances 

 constitute a new implement of research in consequence of the rays 

 they emit, and they ha^-e actively contributed to the development 

 of the theory of the conduction of gas and of the nature of the 

 electron. By their numerous chemical and physiological effects, 

 and their possible influence on meteorology, these substances extend 

 their sphere of action in the domain of all the science of nature ; and 

 it is probable that their importance for the development of science 

 will go on increasing. Finally, it has been shown that there is 

 nothing absurd in supposing that the energy we receive from the 

 sun may be in part, or even in total, due to the presence of radio- 

 active bodies which it may contain. 



o Mme. Curie, Revue geuevale ties Sciences, January 18!;)9, and Revue Scien- 

 tifique, July 1900. 



