l6o BACTERIA 



into the air in the form of a free gas. This is the last stage 

 of all proteid decomposition, so that wherever putrefaction 

 is going on there is a continual " loss " of an element essen- 

 tial to life. Thus it would appear at first sight that the 

 sum-total of nitrogen food must be diminishing. 



But there are other ways also in which nitrogen is being 

 set free. In the ordinary processes of vegetation there is a 

 gradual draining of the soil and a passing of nitrogen into 

 the sea; the products of decomposition pass from the soil 

 by this drainage, and are " lost " as far as the soil is con- 

 cerned. Many of the methods of sewage disposal are in 

 reality depriving the land of the return of nitrogen which is 

 its necessity. Again, nitrogen is freed in explosions of gun- 

 powder, nitroglycerine, and dynamite, for whatever purpose 

 they are used. Hence the great putrefactive " loss " of 

 nitrogen, with its subsidiary losses, contributes to reduce 

 this essential element of all life, and if there were no method 

 of bringing it back again to the soil, it would seem that 

 plant life, and therefore animal life, would speedily terminate. 



It is at this juncture, and to perform this vital function, 

 that the nitrogen-fixing bacteria play their wonderful part : 

 they bring back the free nitrogen and fix it in the soil. Ex- 

 cepting a small quantity of combined nitrogen coming down 

 in rain and in minor aqueous deposits from the atmosphere, 

 the great source of the nitrogen of vegetation is the store 

 in the soil and subsoil, whether derived from previous ac- 

 cumulations or from recent supplies by manure. 



Sir William Crookes has recently 1 pointed out the vast 

 importance of using all the available nitrogen in the service 

 of wheat production. The distillation of coal in the process 

 of gas-making yields a certain amount of its nitrogen in the 

 form of sulphate of ammonia, and this, like other nitro- 

 genous manures, might be used to give back to the soil 



1 British Association for the Advancement of Science, Bristol, 1898, Presi- 

 dential Address. 



