It had but reached the experimental stage at the commencement 

 of the war. It was at once seen that it was a question of vital 

 importance to Germany to be in a position to make a sufficiency of 

 nitrates for the manufacture of explosives. Much artificial nitrate 

 may have been imported from Norway and Sweden, but Chile, the 

 main source of supply, was soon cut off. 



In the Haber process for the synthesis of ammonia, nitrogen and 

 hydrogen in the proportions of one to three by volume are compressed 

 to 150-200 atmospheres pressure and passed over certain catalytic 

 substances at a temperature of about 500 C. At this temperature 

 and in the presence of these special substances whose exact nature, 

 has, I believe, not been made public,* though it is known that a 

 large number of substances are effective- a certain amount of the 

 nitrogen and hydrogen combine to form ammonia. The gases now 

 pass to a chamber kept at a very low temperature (about 40 C.), 

 where the ammonia liquefies and can be drawn off as liquid or allowed 

 to expand as gas and is then converted into sulphate of ammonia. 

 It can also be converted into nitric acid by the Ostwald process. It 

 was by this means that Germany obtained the large supplies of nitrate 

 for the manufacture of explosives. It has been estimated that the 

 B.A.S.F., who employ this Haber process, produced some 500,000 tons 

 of ammonia per annum ; this, when oxidized, would yield some two 

 million tons of nitric acid at least. 



Such, then, are the more important methods by which the free 

 nitrogen of the atmosphere can be brought into combination by purely 

 chemical means. There remain, however, the biological methods, which 

 so far as agriculture is concerned, are of vast importance. In this case 

 the fixation of nitrogen is brought about by bacteria found in soil. 

 The soil bacteria which are capable of bringing about this nitrogen 

 fixation are of two classes: the one is symbiotic, that is to say, it cannot 

 carry on without the assistance of some other organism. In this 

 particular case the organism necessary as host is a plant of the legu- 

 minous family: beans, berseem, peas, clovers, vetches, etc. It had long 

 been observed that plants of this natural order seemed to be almost 

 independent of manures and also that where beans and wheat happened 

 to be growing side by side, as often happens when wheat is sown after 

 beans and many of the latter have shelled and been left on the field, that 

 the wheat was very much stronger in the immediate neighbourhood 

 of the beans. The cause of this was shown by Hellriegel and Wilfarth 

 and afterwards confirmed in a most complete manner by a long series 

 of experiments carried out by Sir John Lawes and Sir Henry Gilbert 

 at Rothamsted. These bacteria play a most important part in 



* In 1919. F.H. 



