DISCOVERY AND PROPERTIES OF RADIUM 141 



methods have never been surpassed and perhaps not equalled in 

 the history of physical science. 



There is one other point worthy of notice. The periodic 

 scheme is justified completely, notwithstanding necessary 

 qualifications, by the results of this work, for by its use discoveries 

 have been made which, without such a guide, would probably 

 have remained unsuspected for ever. 



The new elements are as follows, and they will be found in the 

 table (p. 127) in their several positions. 



Name. Symbol. Atomic or molecular weight. 



Helium ..He .. 4-0 \ 



Neon .. .. Ne .. 20-2 I Most 



Argon ..A .. 39-88 > recent 



Krypton .. Kr .. 82-92 j estimates. 



Xenon Xe 130-2 / 



CHAPTER VII 

 DISCOVERY AND PROPERTIES OF RADIUM 



WHILE all this work was going on in England researches were 

 proceeding in France which were destined to lead to other 

 surprising discoveries. The X-rays had been discovered by 

 Professor Rontgen of Wiirzburg in the autumn of 1895, and 

 early in 1896 Henri Becquerel, Professor of Physics in the 

 Museum of Natural History in Paris, set to work to examine the 

 radiations emitted by phosphorescent bodies of all kinds, in the 

 expectation that the luminous rays might be accompanied by 

 invisible but penetrating radiation, identical with or similar to 

 the Rontgen rays. Observations on the radiating properties of 

 the salts of uranium led him to the discovery that these sub- 

 stances possess the property of affecting a photographic plate. 

 In the first experiments the uranium compound was exposed to 

 the rays of the sun while the sensitised plate was protected by 

 black paper and a sheet of metallic aluminium. 



A few days later he discovered, almost by accident, that 

 exposure of the whole to light was unnecessary, and that if the 

 crystal was kept attached to the plate long enough a very strong 

 impression was obtained in the dark. It thus appeared that the 

 phenomenon could not be attributed to luminous radiations 



