RADIUM. 197 



of radio-activity. It may be remarked that, contrary to the behavior 

 of ordinary gases, the emanation spontaneoasly disappears when con- 

 tained for a sufficient time in a sealed tube. 



The quantity of emanation diminishes by a lialf in four days, and this 

 time constant is characteristic of the emanation of radium. 



IX. 



Having- briefly enumerated the principal properties of radium, it is 

 proper to recall in a few words the origin of its discovery, in which 

 Mme. Curie has had a very great share. '^^ 



Experiments with the substances separated from uranium and tho- 

 rium had showed that the radio-activity is an atomic property which 

 always accompanies the atoms of these simple substances. The radio- 

 activit}' of a complex substance is generally greater the larger the pro- 

 portion of the radio-active metal contained in the compound. Certain 

 ores of uranium, as pitchblende, chalcolite, and camotite, have, how- 

 ever, a radio-activity superior to that of metallic uranium. We there- 

 fore questioned whether these minerals might not contain in minute 

 proportion some substances still unrecognized and far more radio-active 

 than uranium, and we searched by chemical methods for the hj'po- 

 thetical substances, alwaj's guided by the radio-activity of the sub- 

 stance treated. 



Our anticipations were verified ])y the results. Pitchblende con- 

 tains new radio-active substances, but in an excessively minute pro- 

 portion. A ton of pitchblende, for example, contains a quantit}" of 

 radium on the order of one-tenth of a gram. In these conditions the 

 preparation of radium salts is very tedious and costly. A ton of ore 

 furnishes some kilograms of radiferous l)ariuni bromide, from which 

 the radium is extracted by a series of fractionations. 



During the separation of radium, Demaryay, whose recent death is 

 much deplored, was so good as to examine spectroscopicalh" the prod- 

 ucts which we prepared. This cooperation was most valuable to us, 

 for at the conclusion of our research the spectrum analysis confirmed 

 our anticipations and furnisliod the proof that the radio-active barium 

 which we had separated from pitchblende contained a new element. 

 Demar^'ay made the first investigation of the spectrum of radium.* 



Radium has a vory sensitive spectrum reaction — indeed, quite as 

 sensitive as that of ])arium. The presence of radiiau may be detected 

 spectroscopically in radiferous barium containing only one ten- 

 thousandth of radium; hut the radio-activity of radium gives a reac- 

 tion 10,000 times as sensitive still. An electrometer ordinarih' well 

 insulated enal)les the observer to detect readily the presence of radium 



« Mme. Curie, These A, la Faculte des Sciences, Paris, 1903. 

 SDemaryay, C. R., December, 1898, and July, 1900. 



