RADIUM. « 



By J. J. Thomson. 



The discovery by Monsieur and Madame Curie that a samph' of 

 radium oivos out sufficient energy to melt half its weight of ice per 

 hour has attracted attention to the question of the source from which 

 the radium derives the energy necessary to maintain the radiation; 

 this prol)lem has l)een l^efore us ever since the original discovery by 

 Becquerelof the radiation from uranium. It has been suggested that 

 the radium derives its energy from the air surrounding it; that the atoms 

 of radium possess the facult}' of at)stracting the kinetic energy from the 

 more rapidl}^ moving air molecules while they are able to retain their 

 own energy when in collision with the slowly moving molecules of air. 

 I can not see, however, that even the possession of this property would 

 explain the behavior of radium; for imagine a portion of radium 

 placed in a cavity in a ))lock of ice. The ice around the radium gets 

 melted. Where does the energy for this come from ? By the hypothesis 

 there is no change in the energ}' of the air radium system in the cavity, 

 for the energy gained l)y the radium is lost l)vthe air, while heat can 

 not flow into the cavit}^ from outside, for the melted ice around the 

 cavity is hotter than the ice surrounding it. 



Another suggestion which has been made is that the air is traversed 

 by a very penetrating kind of Becquerel radiation, and that it is the 

 a))sorption of this radiation that gives the energy to the radium. We 

 have direct evidence of the existence of such radiation, for McClennan 

 and Burton have recently shown that the ionization of a gas inside a 

 closed vessel is diminished by inunersing the vessel in a large tank full 

 of water, suggesting that part, at any rate, of the ionization of the gas 

 is due to a radiation which could penetrate the walls of the vessel, but 

 which was stopped to an apprecial)le extent by the water. To explain 

 the heating eflect observed with radium, the absorption of this radia- 

 tion by radium nuist be on an altogether different scale from its 

 absorption b}' other metals. As no direct experiments have been made 

 on radium, it is possible that this may be the case; it is not, however, 

 what we should expect frjom the experiments which have been made 

 on the absorption of this radiation by other metals, for these experi 

 ments haye shown that the absorption depends solely upon the density 



«Rei>rinte(l from Natun', London, No. 174.S, vol. (>7, Apr. 30, 1903, pp. (501-602. 

 SM i!M)3 U IW) 



