198 PHYSICAL SCIENCE 



bromide newly precipitated from solution, and has 

 shown that it grows pari passu w^ith the radio- 

 activity as measured electrically — a method which, 

 as we have seen, depends chiefly on the a radiation. 



The greater part of the radiation coming from 

 a solid radium compound is emitted by the stored 

 emanation and its products, the active deposits. 

 The emanation can be extracted only in such 

 minute quantities that, except in most exceptional 

 conditions, its radio-activity alone reveals to us 

 its existence. As we have seen, the emanation 

 is of the nature of a dense gas, half of any 

 quantity of which would be transformed into 

 other substances in about four days. Owing to 

 this process of change, only a limited amount 

 of emanation can be obtained from a given 

 quantity of radium, and the bubble which can 

 be evolved from the small supply of radium 

 possessed by any experimenter is too minute to 

 be visible, except by the most refined and sensitive 

 methods of investigation. Could a cubic inch 

 of the radium emanation be obtained, the radia- 

 tion from it would be so powerful that the vessel 

 used to contain the gas would, in all probability, 

 be fused instantly. 



By the methods we have already described, it 

 is possible to determine the mass and the velocity 

 of the projected particles, and therefore to calculate 

 their kinetic energy. From the principles of the 

 molecular theory, we know that the number of 

 atoms in a gram of a solid material is about 

 lO^^ Eight successive a ray stages in the dis- 

 integration of radium have been recognised ; and, 

 since each of these involves the emission of one 

 particle, the total energy of radiation which i 



