102 THE NITROGEN PEOBLEM. 



carefully prepared and protected, which necessitates the 

 purification, in the first instance, of the gases used. The first 

 cost and maintenance of the plant used is large, and the 

 preparation and purification of the gases, especially hydrogen, 

 are also costly items, but, in spite of these, ammonia can be 

 produced at a cheaper rate by this process than in any other 

 way. The ammonia obtained may be marketed as such or 

 treated with acids to form salts or oxidised by means of air 

 to form nitric acid and then ammonium nitrate. 

 III. — The C yananude Process. 



In 189-1: calcium carbide was produced on the large scale 

 by the process discovered independently by Moissan anfl 

 Wilson. In the following year Frank and Caro, in attempting 

 to prepare potassium cyanide for gold extraction by the 

 Macarthur-Forrest process, passed nitrogen over barium 

 carbide mixed with soda heated to 700° C. to 800° C, and 

 found that about 30 per cent, of barium cyanide was formed 

 and, in addition, 45 per cent, of barium cyanamide. When 

 the price of cyanide fell owing to the Boer War, they used the 

 cheaper calcium compound, and observed that cyanamide alone 

 was formed. This substance, when treated with water, yielded 

 ammonia, and after some nreliminary trials quickly found 

 extensive use as a fertiliser under the name nitrolime or 

 lime nitrogen. At first, however, ammonium sulphate Avas 

 prepared from it by placing it on trays in a tower up whicli 

 steam was passed, tbe ammonia set free being subsequently 

 dissolved in sulphuric acid. At high pressures this reaction 

 with steam is almost quantitative, and is stated to give a 

 95 per cent, efficiency on the large scale. 



The raw materials used in this process are lime, coke and 

 nitrogen — substances which can be obtained in abundance and 

 plenty. This, coupled with the fact that the raw product can 

 be used directly as a fertiliser, and also that the power require- 

 ments are small, has made this the most popular form of 

 nitrogen fixation, large factories being established in France, 

 United States of America, Canada, Italy, Scandinavia and 

 Japan, as well as in Germany. The nitrogen is now almost 

 universalh' obtained by distilling liquid air according to the 

 inventions of Claude and Linde, based on the dift'erences of 

 the boiling points of nitrogen -196° C, air -194° C, and 

 oxygen -188° C. The nitrogen so obtained is guaranteed not 

 to contain more than ()-4 per cent, oxygen, and this small 

 amount has caused no difficulty in practical working. It may 

 be mentioned also that this method of obtaining on the large 

 scale the gases of the atmosphere in liquid form and separating 

 their constituents by fractionation is also one of the notable 

 achievements of this century, based primarily on the 

 researches of Professor Andrews on carbon dioxide in 1869, 

 which first provided tjie key for the solution of the problem. 

 He proved that all gases must be reduced in temperature lielow 

 a certain point, known as the critical temperature, and when 

 this Avas done pressure alone would serve to bring about the 

 change to the liquid state. As a result of this discovery all 

 known gases were reduced to lic|uids on a laboratory scale, 



