CHAP, vii.] NITRIFICATION 121 



the ground, yet we do not know of any means by which 

 these nitrogen compounds can be converted into nitrates 

 in the laboratory except by most difficult and involved 

 processes. The whole history of nitrates in the soil will, 

 in fact, repay a little study. It has been known from 

 time immemorial that nitre in some form or other is a 

 product of the soil, and in India the men of a whole 

 caste or guild make their living by gathering nitre. They 

 scrape away the surface soil from time to time in the 

 neighbourhood of the villages just beyond the point where 

 the drainage finds an outlet, extract it with hot water, 

 and then after clarification concentrate the solution thus 

 obtained, the result being the nitrate of potash, nitre, or 

 saltpetre of commerce. Similarly, in Egypt the native 

 cultivator knows that the earth from deserted villages 

 — ^the dust, in fact, into which the old mud-built habita- 

 tions resolve themselves — forms a valuable fertiliser and 

 will yield nitre when extracted with water. Again, nitre 

 used to be obtained in Europe from the earth of cellars, 

 stable floors, and other fairly dry places near habitations; 

 in later times it was expressly manufactured by building 

 up nitre beds — mixtures of earth with a certain amount 

 of carbonate of lime and organic matter, over which 

 urine and similar liquids containing compounds of 

 nitrogen were poured from time to time. The bed was 

 protected from the rain, was kept moist but not wet, and 

 after a period of from three to five years it was extracted 

 with water and yielded a solution of nitrate of lime, 

 which could be afterwards converted into nitrate of 

 potash for gunpowder-making. This well-known process 

 of nitrification represents the fact that the soil is able to 

 bring about a change in the organic compounds of 

 nitrogen and convert them into nitrates, either nitrate of 

 lime or nitrate of potash, according to the base which 

 happens to be present. Long before the mechanism of 



