THE NITRIFYING BACTERIA. 511 



the soil quickly disappear, and ultimately only the nitri- 

 fying 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 Winograd- 

 sky 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 spe- 

 cific gravity of 1.02, remains fluid and can be preserved 

 in flasks in this condition. (Kuhne.) Gelatinization oc- 

 curs after the addition of certain salts to such a solution, 

 and will be more or less complete according to the pro- 

 portion of salts added. The salts that have given the 

 best results and the method of mixing them are as fol- 

 lows : 



r Ammonium sulphate 0.4 gramme. 



a \ Magnesium sulphate 0.05 " 



( Calcium chloride trace. 



( Potassium phosphate 0.1 gramme. 



b < Sodium carbonate ... 0.6 to 0.9 " 



( Distilled water 100 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 little, until the proper degree of consist- 

 ency is obtained (that of ordinary nutrient gelatin). 

 This part of the process is best conducted in a culture- 

 dish. If it is desired to separate the colonies, as in an 

 ordinary plate, the inoculation and mixing of the mate- 

 rial introduced must be done before gelatinization is 



