314 A CENTURY OF SCIENCE 



two. The rare earths are peculiar from the fact that 

 many of them are always found mixed together in the 

 minerals containing them, and also from the circum- 

 stance that most of them are remarkably similar in their 

 chemical reactions and consequently exceedingly difficult 

 to separate from each other. In many cases multitudes 

 of fractional precipitations or crystallizations are needed 

 to obtain pure salts of a number of these metals. The 

 solutions of the salts of several of these elements give 

 characteristic absorption bands when examined spectro- 

 scopically by the use of transmitted light. 



No important practical application has been found for 

 any of these earthy oxides, except that about one per cent 

 of cerium oxide is mixed with thorium oxide in incandes- 

 cent gas-mantles in order to obtain greatly increased 

 luminosity. 



The Inactive Gases. As long ago as 1785, Cavendish, 

 that remarkable Englishman who first weighed the world 

 and first discovered the composition of water, actually 

 obtained a little argon in a pure condition by sparking 

 atmospheric nitrogen with oxygen converting it into 

 nitric acid (another discovery of his) and absorbing the 

 excess of oxygen. The volume of this residual gas as 

 estimated by him corresponds very closely to the volume 

 of argon in the atmosphere, as now known. 



It was more than a century later, in 1894, that Rayleigh 

 and Ramsay discovered argon in the air. Lord Rayleigh 

 had found that atmospheric nitrogen was about one-half 

 per cent heavier than chemical nitrogen, a fact which led 

 to the investigation. It was only necessary to repeat 

 Cavendish's experiment on a large scale, or to absorb 

 oxygen with hot copper and nitrogen with hot mag- 

 nesium, in order to obtain argon. The gas attracted 

 much attention, both on account of having but a single 

 atom in its molecule, and particularly because it failed to 

 enter into chemical combination of any kind. This gas 

 has been used of late for filling the bulbs of incandescent 

 electric lamps in cases where a gas-pressure without 

 chemical action is desired. 



In 1890 and 1891, Hillebrand published in the Journal 

 40, 384, 1890: 42, 390, 1891) a series of analyses of the 

 mineral uraninite and reported in some samples of the 



