432 BAOTEBIOLOGY. 



development, those that were originally introduced with 

 the soil quickly disappear, and ultimately only the nitri- 

 fying organisms remain. These are to be seen as an 

 almost transparent film attached to the clumps and gran- 

 ules of magnesium carbonate on the bottom of the flask. 

 For their cultivation upon a solid medium he 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). By the addition of certain 

 salts to such a solution gelatinization occurs, 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 : 



r Ammonium sulphate . . . 0.4 gramme. 

 a < Magnesium sulphate . . 0.05 " 



(. Calcium chloride trace. 



{Potassium phosphate . . . .0.1 gramme. 



Sodium carbonate . . . 0.6 to 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 (6) 

 in the remaining 60 c.c, in separate flasks. 



Each flask is then sterilized with its contents, which 

 after cooling are mixed together. This represents the 

 solution of mineral salts that 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 the 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 



