246 DISCOVERY en. 



As about two million tons of nitrate of soda are 

 shipped from South American ports annually, there is 

 a prospect of the ultimate exhaustion of the supply. 

 In the earth's atmosphere, however, there is practically 

 an inexhaustible store of nitrogen, and by methods 

 based upon the experiments of Priestley and Cavendish 

 the nitrogen of the air is now being used in the manu- 

 facture of nitrates or other compounds which can take 

 the place of the natural saltpetre. 



In 1828, a Swedish chemist, J. J. Berzelius, whose 

 investigations have been a lamp unto the path of 

 succeeding generations, discovered a rare element to 

 which the name Thorium was given ; he also carried out 

 noteworthy researches on another rare earth Cerium- 

 discovered by a German chemist, M. H. Klaproth, some 

 years earlier. Until recent years these substances were 

 rarely mentioned even in text-books of chemistry, and 

 their existence was only regarded as of academic interest, 

 yet, by their use in the manufacture of the incandescent 

 mantle in 1885, they saved coal-gas from being super- 

 seded by electricity. 



The German chemist, Auer von Welsbach, while inves- 

 tigating the rare metals of the thorium group, found that 

 certain compounds became luminous when held in the 

 flame of a Bunsen burner. He dipped a piece of fabric 

 in a solution of a thorium compound and then held it in 

 the Bunsen flame, with the result that the fabric was 

 consumed and a coherent residue of thoria, which gave 

 a bright light, remained. This was the beginning of 

 the incandescent mantle now in common use everywhere. 

 To increase the luminosity of the mantle, a small 

 quantity of some other substance has to be added to the 

 thorium compound ; and by experiment von Welsbach 

 found that the best result is obtained by the addition 



