CHAPTER XLVII 

 RADIUM, ATOMIC ENERGY, AND ATOMIC STRUCTURE 



The Discovery of Radium. In 1896 Henri Becquerel dis- 

 covered that a crystal of a salt of uranium could, in the dark, 

 reduce the silver bromide on a photo- 

 graphic plate, even when a sheet of black 

 paper (impervious to light) was placed be- 

 tween. Evidently a radiation, different 

 from light, was given out by the salt. 

 Next he discovered that an electrometer 

 (Fig. 119), in which the go.d leaves had 

 been caused to separate by charging with 

 electricity, lost its charge rapidly when a 



salt of uranium was brought near to the knob connected with the 

 leaves. Evidently the salt rendered the air a conductor (" ionized " 

 the air), and this permitted the escape of the electricity. These 

 discoveries, in the hands of a multitude of observers, have led to 

 the development of an entirely new branch of our science, namely 

 radio-chemistry. 



The radioactivity of every pure uranium compound is pro- 

 portional to its uranium content. The ores are, however, rela- 

 tively four times as active. This fact led M. and Mme. Curie, 

 just after 1896, to the discovery that the pitchblende residues, from 

 which practically all of the uranium had been extracted, were 

 nevertheless quite active. About a ton of the very complex resi- 

 dues having been separated laboriously into the components, it 

 was found that a large part of the radioactivity remained with the 

 sulphate of barium. From this a product free from barium, and at 

 least one million times more active than uranium, was finally 

 secured in the form of the bromide. The nature of the spectrum 

 and the chemical relations of the element, now named radium, 



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