THE BECQUEREL RAYS 141 



the name radio-thorium was applied by its discoverer, 

 Otto Hahn. 



Besides the properties already mentioned, radium, and 

 presumably the others, have the curious property of 

 changing a spark-discharge from an electric machine or 

 a Ruhmkorffs coil into a violet glow-discharge; the 

 interposition of a piece of lead, however, re-establishes 

 the spark-discharge ; and if barium bromide containing 

 radium be held on the forehead between the closed eyes 

 in a dark room, a distinct luminous haze is visible after 

 a few seconds. The actinium rays, indeed, are said to be 

 100,000 times as powerful as those of uranium. Very 

 powerfully radio-active preparations of barium chloride 

 and bromide are now manufactured by various firms by 

 processes devised by Madame Curie and by Professor 

 Giesel. 



A new light has been thrown on all these phenomena 

 by Professor Rutherford, who has found that thorium 

 compounds give out an ' emanation,' which may be likened 

 to one of Boyle's ' exhalations of the terraqueous globe.' 

 Dr. Russell had previously discovered that photographic 

 plates are affected by hydrogen dioxide vapour, which 

 appears to be produced in small amount under the most 

 varying conditions ; but Rutherford's exhalations persisted 

 under treatment which would have been fatal to hydrogen 

 dioxide ; moreover, these emanations rapidly discharge 

 electrified bodies, a property which hydrogen dioxide does 

 not possess. The existence of such emanations (of which 

 more hereafter) must be borne in mind in forming a 

 judgment of the statements made about these various 

 radiations. 



Radio-active substances can communicate transitory 

 radio-activity to all kinds of matter, metals, glass, paper, 

 etc., which then for a short time possess radio-activity 

 equal to ninety times that of uranium. They lose the 



