SALTPETRE. 383 



nitrates, ifcc., formed in the interior appear like wall-saltpetre 

 on the surface of the mass, and gradually increase to form a crust 

 which is richer in nitrates than the interior of the heap. The 

 crude lye obtained therefrom by lixiviation is treated by adding a 

 potassium salt in order to convert the nitrates of calcium, mag- 

 nesium, and sodium into potassium nitrate, the crude saltpetre 

 thus produced being then purified in refineries. 



The elucidation of the optimum external conditions for in- 

 fluencing nitrification has been attempted by numerous investi- 

 gators, and a few of their results will now be given. J. DUMONT 

 and J. CROCHETELLE (II. and III.) found that the chlorides of 

 potassium and calcium injuriously affect nitrification, whereas 

 the carbonates of these metals, and also potassium sulphate, act 

 beneficially. From what has already been stated it will be evident 

 that the merely faint (or altogether inoperative) activity of the 

 nitrifying bacteria in soils poor in calcium carbonate (e.g. sour 

 meadow-land) can be stimulated by the addition of the said car- 

 bonate. On this point a few experiments have been made by 

 J. DUMONT and J. CROCHETELLE (I.). The kind of acid with 

 which the ammonia is combined must not be regarded as unim- 

 portant, Hueppe and Winogradsky having noticed that as after- 

 wards shown by the special experiments of 0. LOEW (VI.) 

 nitrifying bacteria do not attack ammonium formate at all, and 

 that the oxalate is acted upon only very imperfectly, and with 

 .great difficulty. 



