JOSEPH BLACK: HIS LIFE AND WORK 87 



electroscope, and since Madame Curie discerned one of 

 the causes of the discharge by uranium ore, namely, the 

 existence in it of a new element, radium, and since Ruther- 

 ford and Soddy's isolation of the gases evolved from 

 radium and from thorium, that a new and more sensi- 

 tive instrument has been placed at the disposal of 

 chemists in the electroscope. We are at the beginning of 

 a new era. Every discovery of a new principle of research 

 heralds a new departure; and the compound nature of 

 many of the so-called elements begins to appear from 

 their electrical behaviour, in much the same manner as 

 Black demonstrated the decomposability of compounds in 

 the year 1752. 



