118 Generating Economic Cycles 



ing to the principle of reversibility, to inquire whether 

 the converse relation may not hold true. If, for ex- 

 ample, an electric current can produce a magnet, it is 

 reasonable to suppose, as Faraday showed, that a mag- 

 net can produce an electric current. After the observa- 

 tion was made that cathode rays, positive rays, and 

 Roentgen rays illuminate a fluorescent screen, the 

 principle of reversibihty obviously suggested that 

 those substances which make good screens may pos- 

 sibly emit rays that are similar to the positive, cathode, 

 and Roentgen rays. 



Henri Becquerel was one of the first to enter this 

 field which has been so successfully exploited by Pro- 

 fessor and Madame Curie and Professor Rutherford. 

 The history of researches into radio-activity is recent 

 and well-known. The central discovery, however, 

 needs to be connected with what has been said about 

 the radiations attending the discharge of electricity in 

 rarefied gases. Three principal kinds of rays, known 

 respectively as a-, jS-, and 7-rays, have been derived 

 from radio-active substances, and these rays have their 

 analogues in the radiations of the discharge tube. 

 The a-rays are positively electrified and resemble in 

 general characteristics the positive rays of the dis- 

 charge tube. The jS-rays are negatively electrified and 

 resemble cathode rays. The 7-rays resemble the 

 Roentgen rays. 



These discoveries in electricity and in the nature of 

 matter have produced a veritable renaissance in astro- 

 nomical speculation. " La decouverte des electrons et 

 le succes croissant dc la conception corpusculaire de 



