84 CHEMICAL DISCOVERY AND INVENTION 



carbide placed in a suitable generator, and connected with a gas- 

 holder in which the gas is collected over water. 



Since 1903, however, calcium carbide has received application 

 in a new direction, having been found to absorb nitrogen when 

 heated to about 1000 C., in the presence of small quantities of 

 calcium chloride or some other salts. The product is the calcium 

 salt of cyanamide, CN.NCa, and is known commercially as 

 " nitrolime." It is a valuable nitrogenous manure. 



The application of the electric furnace to such operations as 

 the manufacture of steel is obviously a subject of the highest 

 importance, but is outside the programme of the chemist. 



The use of electrical methods for bringing about the com- 

 bination of atmospheric nitrogen with oxygen, and the fixation 

 of nitrogen and its oxides into solid compounds will be described 

 in a later chapter. 



Cooling. Ice is an indispensable agent for use in the labora- 

 tory, especially in connection with the reactions in which carbon 

 compounds are concerned. In very many cases the heat which 

 is produced by chemical change goes on accumulating in the 

 materials which have been mixed together until the temperature 

 of the whole is such as either to cause the volatile ingredients to 

 boil and evaporate away, or to give rise to new changes which 

 may become violent and uncontrollable. The result is that the 

 desired product is not secured and dangerous explosions may result. 



The temperature of melting ice is always at C., but lower 

 temperatures are easily obtained by mixing it with due pro- 

 portions of very soluble salts. With common salt the tempera- 

 ture falls to the zero of the Fahrenheit scale (-17-7), and a few 

 pounds weight of such a mixture will keep a temperature of 

 -12 to -15 for a long time. 



But much of the experiment during the last twenty years has 

 required the employment of temperatures far below such limits, 

 and even mixtures such as solid carbon dioxide in ether or 

 alcohol, which gives a temperature of about -75 C., are not low 

 enough. Moissan in his experiments which resulted in the 

 isolation of fluorine, made use of boiling methyl chloride as a 

 frigorinc agent maintaining a fairly steady temperature of -23 C. 



The liquefaction of atmospheric air in quantity has placed in 

 the hands of the physicist and chemist an agent which has 

 indirectly furnished the means of reducing the remaining more 

 refractory gases, hydrogen and helium, to the liquid state. This 



