254 SECTIONAL ADDRESSES 
metric tons). There has, of course, been a fall since then, but this is due 
very largely to the prevailing depression. Formerly, superphosphate was 
considered an acid manure and its continued use was supposed to deplete 
the soil of lime and to increase its acidity. A large amount of experi- 
mental work has been carried out in recent years, and the result is 
to show conclusively that the objections to the use of the so-called 
physiologically acid manures have been the result of misconceptions or 
possibly even misrepresentations. ‘The use of superphosphate does not 
generally increase the acidity of the soil. 
Nitrogenous Fertilisers —Attention has already been directed to the far- 
reaching effects of Leibig’s reports to the British Association in 1843 and 
1847. Inthe year 1898 the Association was again to take a prominent part 
inthe development of agriculture. ‘The President that year was Sir William 
Crookes, who devoted his address to showing that if the rate of increase then 
assumed of the world’s population was correct, the world would be faced 
with a wheat famine in the not far distant future. He pointed out also 
the necessity, if we were to increase the production of wheat, of the 
increased use of ammonium salts and nitrates as fertilisers. As regards 
nitrogenous fertilisers, he showed that we were living on our capital 
of combined nitrogen compounds and that there was also the danger of a 
nitrogen famine to be faced. The remedy he suggested was to devise 
methods for ‘ fixing’ or bringing into combination the nitrogen of the 
atmosphere, and he actually sketched methods and estimated costs of 
effecting this combination by electrical means. As far back as 1784 
Cavendish had shown that oxygen and nitrogen could be made to combine 
under the influence of the electric spark. Many years were to elapse, 
however, before a practical commercial method was evolved. ‘The 
earlier methods were electrical in character and were developed in 
Norway and Italy, where cheap supplies of electrical energy were available. 
These methods have been more or less superseded, and ammonia is now 
manufactured by a synthetic method on an enormous scale at the works 
of the Imperial Chemical Industries, Ltd., at Billingham. ‘The method 
used is a modification of the Haber-Bosch process. The practical difh- 
culties which had to be faced were great ; of these I need only mention 
the problem of working at pressures of over 200 atmospheres—i.e. over 
3,000 lb. per square inch—and at an elevated temperature, to enable you 
to realise some of the difficulties which had to be overcome. ‘The success- 
ful development of the method is certainly one of the greatest triumphs 
of chemistry and engineering in modern times. By this process, which 
incidentally dispenses with the use of sulphuric acid, sulphate of 
ammonia can now be prepared more cheaply than from gas liquor 
where the ammonia is obtained as a by-product. 
By the oxidation of ammonia to nitric acid by means of a suitable 
catalyst ammonium nitrate can be prepared, and by mixing this with calcium 
carbonate a valuable fertiliser, known commercially as ‘ nitro-chalk,’ is 
manufactured. 
Concentrated Complete Fertilisers —One of the most interesting develop- 
ments of the synthetic ammonia industry has been the manufacture of 
concentrated complete fertilisers containing nitrogen, phosphates and 
