Wells & Foote — One Hundred Years of Chemistry. 301 



the production of nitric acid depends upon the oxidation 

 of ammonia by air with the aid of a contact substance, 

 such as platinized asbestos. 



The production of ammonia, which was very small a 

 hundred years ago, has been vastly increased in connec- 

 tion with the development of the illuminating-gas indus- 

 try and the employment of by-product coke ovens. This 

 substance is very extensively used in refrigerating 

 machines and also in a great many chemical operations, 

 including the Solvay soda-process. Ammonium salts 

 are of great importance also as fertilizers in agriculture. 

 The conversion of atmospheric nitrogen into ammonia 

 on a commercial scale is a recent achievement. It has 

 been accomplished by heating calcium carbide, an elec- 

 tric-furnace product made from lime and coke, with nitro- 

 gen gas, thus producing calcium cyanamide, and then 

 treating this cyanamide with water under proper condi- 

 tions. Another method devised by Haber consists in 

 directly combining nitrogen and hydrogen gases under 

 high pressure with the aid of a contact substance. 



Leblanc's method for obtaining sodium carbonate from 

 sodium chloride by first converting the latter into the 

 sulphate by means of sulphuric acid and then heating the 

 sulphate with lime and coal in a furnace was invented 

 as early as 1791, but it was not rapidly developed and did 

 not gain a foothold in England until 1826 on account of a 

 high duty on salt up to that time. Afterwards the 

 process flourished greatly in connection with the sul- 

 phuric acid industry upon which it depended, and with 

 the bleaching-powder industry which utilized the hydro- 

 chloric acid incidentally produced by it, and, of course, 

 in connection with soap manufacture and many other 

 industries in which the soda itself was employed. 



About 1866 the Solvay process appeared as a rival to 

 the Leblanc process. This depends upon the precipita- 

 tion of sodium bicarbonate from salt solutions by means 

 of carbon- dioxide and ammonia, with the subsequent 

 recovery of the ammonia. It has displaced the older 

 process to a large extent, and it is carried on extensively 

 in this country, for instance, at Syracuse, New York. 



Other processes for soda depend upon the electrolysis 

 of sodium chloride solutions. In this case caustic soda 

 and chlorine are the direct products, and the chlorine 

 thus produced and liquified by pressure in steel cylinders, 

 has become an important commercial article. 



