THE NITRIFYING BACTERIA 577 



from over the surface of the layer of magnesium carbonate 

 and transferred to a second flask, and similarly after four 

 or five days from this to a third flask, and so on. As this 

 medium does not offer conditions favorable to the growth 

 of bacteria requiring organic matter for their development, 

 those that were originally introduced with the soil quickly 

 disappear, and ultimately only the nitrifying organisms 

 remain. These are seen as an almost transparent film 

 attached to the clumps and granules of magnesium carbonate 

 on the bottom of the flask. 



For their cultivation upon a solid medium Winogradsky 

 employs a mineral gelatin, the gelatinizing principle of 

 which is silicic acid. A solution of from 3 to 4 per cent, 

 of silicic acid in distilled water, and having a specific gravity 

 of 1.02, remains fluid and can be preserved in flasks in this 

 condition. (Kiihne.) Gelatinization occurs after the 

 addition of certain salts to such a solution, and will be more 

 or less complete according to the proportion of salts added. 

 The salts that have given the best results and the method 

 of mixing them are as follows: 



Ammonium sulphate 0.40 gram 



Magnesium sulphate . . 05 gram 



Calcium chloride . trace. 



Potassium phosphate . . . 10 gram 



Sodium carbonate . 0.6 to 0.90 gram 



Distilled water . . 100.00 c.c. 



The sulphates and chloride (a) are mixed in 50 c.c. of the 

 distilled water, and the phosphate and carbonate (b) in 

 the remaining 50 c.c, in separate flasks. 



Each flask is then sterilized with its contents, which after 

 cooling are mixed; the mixture representing the solution 

 of mineral salts is to be added to the silicic acid, little by 

 37 



