1912] on Vital Effects of Radium and other Rays. 299 



18i)5, Henri Becqiierel proceeded to investigate the ])ossibility of 

 tluorescent substances giving out invisible rays similar to those of 

 Rontgen. Very fortunately, as it turned out, he selected uranium for 

 his purpose. Wrapping a photographic plate in black paper, so that 

 it was completely shielded from all liglit, he placed npon it a salt of 

 uranium, his idea being that the fluorescence induced by the light in 

 the uranium compound might give rise to invisible rays like those 

 from a Crookes tube. He found that the photographic plate was, 

 indeed, blackened underneath the salt, but that such an effect was 

 produced equally well without the uranium being exposed to light. 

 In short, it became evident that the property of emitting these 

 invisible rays was a property inherent in the uranium element itself. 

 Further investigation proved that these rays from uranium, like the 

 ic-rays, had the power of discharging electrified bodies. 



Radio- Activittj .—T\us> discovery of Becquerel was really the dis- 

 covery of that which we now know as radio-activity. The term 

 " radio-active," as applied to elements which spontaneously emit 

 invisible rays that possess the power of passing through opaque sub- 

 stances, has now come into the general vocabulary. The rays of 

 radio-active substances also have the power of discharging electrified 

 bodies, of acting on a photographic plate, and of making certain sub- 

 stances fluoresce. They cause air and other gases to Ijecome tem- 

 porarily conductors of electricity, and, finally, they generate heat 

 spontaneously. 



[Show pitchblende.] 



Becquerefs discovery was also the starting-point for the investiga- 

 tions of the Curies, which resulted in the separation by chemical pro- 

 cesses of the various active substances in a large quantity of residues 

 of the mineral pitchblende. This mineral was found to be more 

 radio-active than the uranium it contained. Madame Curie, after 

 separating a substance which she called polonium, succeeded ulti- 

 mately in isolating the substance which is now known as radium, and 

 which proved to be two million times more radio-active than a 

 corresponding amount of uranium. Radium, chemically considered, 

 is an ordinary element, very similar to barium or calcium and pro- 

 ducing similar salts. The sulphate, chloride, and now more generally 

 the bromide of radium, are the salts used. MetaUic radium has been 

 isolated by Madame Curie. 



[Show calcium which resembles Radium.] 



The Radium Atom. — Up to the time of the discovery of radium 

 the atom had l)een looked upon as indivisible. Rutherford and Soddy 

 found that radium was disintegrating, and Sir William Ramsay, with 

 Mr. Soddy, investigating this disintegration of radium, proved that 



