604 APPLICATION OF METHODS OF BACTERIOLOGY 



To each flask containing 100 c.c. of this fluid is added from 

 0.5 to 1 gram of basic magnesium carbonate suspended in 

 a little distilled water and sterilized by boiling. One of the 

 flasks is then to be inoculated with a minute portion of the 

 soil under investigation, and after four or five days a small 

 portion is to be withdrawn, by means of a capillary pipette, 

 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 0.05 gram 



Calcium chloride trace. 



Potassium phosphate 0.10 gram 



b\ Sodium carbonate 0.6 to 0.90 gram 



Distilled water 100.00 c.c. 



