I I 8 AGRICULTURAL BACTERIOLOGY. 



quently watering it with liquid manure. In this heap occurred 

 the various kinds of nitrogen decomposition already mentioned, 

 and later the nitrification process began. The result was that 

 nitrates were formed in the interior of the heap in large quantity. 

 Eventually the nitrates were extracted by water and converted 

 into nitrate of potassium by the addition of some potassium 

 salt. 



This method of making saltpeter was discovered before 

 science had any idea of the real nature of the process, and it 

 was a practical means of utilizing a part of the nitrogen in the 

 organic substances derived from animals and plants. Whether 

 it was the most efficient means, or more useful than the 

 simple compost heap and manure pile, can hardly be stated. 

 The saltpeter plantations have gone out of existence since the 

 introduction of Chilian saltpeter, but either this or some other 

 means of obtaining saltpeter will inevitably be adopted again 

 when the Chilian mines are exhausted. 



Nitrates are actually formed upon the walls of closets and 

 stables where ammonia fumes are abundant. On such walls 

 may frequently be seen a snow-white mass consisting of cal- 

 cium nitrate. It is the result of nitrification of the ammonia 

 which arises from the manure pile and which unites with oxy- 

 gen and produces nitric acid. The acid combines with the 

 calcium present in the brickwork to form calcium nitrate. 

 The action is an undesirable one from the standpoint of the 

 persistence of the walls, since it produces a corroding action 

 tending to weaken the structure. It may of course be readily 

 prevented by sprinkling the walls with a strong solution of 

 some powerful antiseptic, such as formalin or corrosive subli- 

 mate. 



BACTERIA IN SEWAGE. 



Though a matter of great importance for the hygiene of 

 cities, the problem of sewage is not very closely related to 

 agriculture. There are certain phases of the subject, however, 



