HYDROGEN 245 



volume of which is about \ of the hydrogen. The quantity of 

 air in each can be regulated by means of a pump so that the 

 internal pressure of the whole envelope remains constant and 

 the centre of buoyancy adjusted, so that the balloon and its 

 load retain their relative positions. 



Another application of hydrogen to an entirely different 

 chemical purpose, namely the hardening of oils and fats, does 

 require the hydrogen employed to be free from impurities such 

 as sulphur, phosphorus, and arsenic, even in minute quantities, 

 as they so readily attach themselves to metals. Reasons for this 

 condition of purity in the hydrogen employed in this new 

 industry are given in the chapter relating to " Catalysis." 



Hydrogen, then, is now required on a scale never contem- 

 plated until quite recent times. The first patents relating to the 

 hydrogenation of oils date only from 1903, and it was not till 

 later that the high standard of purity required for this purpose 

 was recognised. 



Another process in which hydrogen may before long be 

 required in large quantity is the production of ammonia syn- 

 thetically by Haber's method, as described in the chapter on 

 " Catalysis." In this case, as in the hydrogenation of oils, the 

 gas employed must be free from sulphurous and other impurities. 



Other uses of hydrogen have developed or expanded of late 

 years, such as the melting of platinum, the working of fused 

 quartz in making apparatus of silica, the autogenous soldering 

 of lead sheet, and the making of joints in lead pipes employed in 

 chemical works. These are found to resist corrosion by acids 

 much more successfully than when solder is used. 



We may now review the principal methods by which hydrogen 

 may be obtained, and from a consideration of the materials and 

 conditions of each some conclusion may be arrived at as to their 

 practicability from the industrial point of view. 



We may at once exclude those processes which are too costly 

 or otherwise objectionable, such as the action of metallic sodium 

 on water or the dissolution of metallic aluminium in caustic 

 soda. These are frequently resorted to in the laboratory, but 

 are unsuitable for use on a large scale. 



The available methods may be ranged under several heads as 

 follows : 



1. The action of metals on acids. 



2. The action of metals on steam at a red heat or on water. 



