6 THE SPIRIT OF THE SOIL 



times have seized it in the form of Carbon dioxide, 

 and changed it in their leaves to the form in which 

 they could assimilate it. The second, Nitrogen, is 

 also present in the air, but in such a form that no 

 plant is able to assimilate it or to combine it so as 

 to render it available for its use. Sir William 

 Crookes put the problem clearly and succinctly. 

 Either the world in thirty or forty years from the 

 date of his address was doomed to suffer from a 

 wheat famine unexampled in the history of the 

 world, or it was incumbent on the man of science 

 to find a practical means of inducing the inert nitro- 

 gen of the air to enter into combination with oxygen, 

 and form the nitrates essential for plant food. 



There was one outstanding classical experiment 

 to guide the chemists in their elucidation of the 

 problem. In 1785 Cavendish discovered that when 

 an electric current was passed through air an acid 

 was formed, and seventy-two years later Bunsen 

 established that the acid so formed was Nitric Acid, 

 the Oxygen and Nitrogen of the air having been 

 induced by means of the energy of the electric dis- 

 charge to enter into combination to form the acid- 

 On this groundwork the chemists set to work, and in 

 a few years' time they had devised a practical method 

 whereby the water-power of the great waterfalls 

 could be used to produce a high-tension electric 

 current. The current was allowed to discharge 

 across a gap, was spread out by means of a powerful 

 magnet into a great disc of blazing, roaring flame, 

 which, on air being forced through it, induced the 

 Nitrogen and Oxygen present to enter into combina- 



