ONE HUNDEED YEARS OF CHEMISTRY 315 



mineral as much as 2-5 per cent of an inactive gas. 

 Hillebrand examined the gas spectroscopically but, just 

 missing an important discovery, he detected only the 

 spectrum lines of nitrogen. Ramsay, in searching for 

 argon in some sort of natural combination, and doubt- 

 less remembering Hillebrand 's work, heated some 

 cleveite, a variety of uraninite, and obtained, not argon, 

 but a new gas. This gave a yellow spectrum-line cor- 

 responding to a line previously observed in the light of 

 the sun's corona and attributed to an element in the sun 

 called helium. Helium, therefore, in 1895 had been found 

 on the earth. This gas is a constant constituent of 

 uranium minerals, as it is produced by the breaking down 

 of radioactive elements. It has been found in very small 

 quantity in the atmosphere, and is the most difficult of all 

 known gases to liquefy, as its boiling point, as shown by 

 Onnes in 1908, is only 4 above the absolute zero. It has 

 not yet been solidified. 



In 1898 Ramsay and Travers, by the use of ingenious 

 methods of fractional distillation and absorption by char- 

 coal, obtained three other much rarer inactive gases 

 from the atmosphere which they called neon, krypton and 

 xenon. 



The inactive gases are all colorless, and as they form 

 no chemical compounds they are characterized by their 

 densities, which give their atomic weights, by their boil- 

 ing points, and by their characteristic Geissler-tube spec- 

 tra. 



The gaseous radium emanation, or niton, belongs also 

 to the inactive group, and it was also collected and 

 studied by Ramsay who was compelled to work with only 

 0-0001 cc. of it, as the volume obtained by heating radium 

 salts is very small. It is an evanescent element, disap- 

 pearing within a few days on account of radioactive dis- 

 integration. Meanwhile it glows brilliantly when lique- 

 fied and cooled to the temperature of liquid air. It has 

 an atomic weight of 222, four units below that of radium, 

 and the difference is considered as due to the loss by 

 radium of an atom of helium in passing into the 

 emanation. 



The Radioactive Elements. The discovery of radium 

 in 1898 by Madame Curie, and the study of that and other 



