TECHNIC BACTERIOLOGICAL EXAMINATION OF SOILS. 287 



bear definite relations to the concentration of the materials used to make the 

 mixture. 



The bacteriologist makes specified dilutions and solutions of media and 

 salts in the same way the chemist does. Further the bacteriologist has to 

 reduce the concentration of the bacteria in all kinds of materials so that an 

 aliquot containing few enough bacteria to be handled with present apparatus 

 and technic can be taken. Water is the usual diluent employed and the con- 

 centration of the bacteria has to be reduced whether the substance under 

 examination is a gas, liquid, or solid, whether it is soluble or insoluble, mis- 

 cible or non-miscible. 



When the substance under bacteriological examination is a liquid the 

 report is usually made of number of bacteria per cc when a solid of the 

 number of bacteria per gram. When the materials are such that specified 

 dilutions of them can be made, the bacteriological dilutions are dilutions of 

 both the materials and the baet3ria. When the materials are solids or non- 

 miscible the bacteriological dilutions are dilutions of the bacteria only. 



Bacteriologically speaking the dilution of bacteria states the number of 

 cubic centimeters of diluent which would contain the number of bacteria 

 in one gram or one cubic centimeter of the original material. 



Throughout this paper the phrases dilution of bacteria per gram of field 

 soil and bacterial dilution have been used. Investigators and texts agree 

 fairly well that the bacteria in soil are intimately associated with the moisture 

 in the soil. Explanations of the finding of living bacteria in frozen soil cluster 

 about discussions of whether the films of moisture surrounding the soil par- 

 ticles are really frozen. The bacteria in soil, when dilutions are made, are 

 diluted for they are distributed through a larger volume of water. Dilutions 

 are based wholly on the volume of water added. Fifty grams of soil and 200 

 cc of water means a 1-4 bacterial dilution of the fift.y grams taken, for each 

 gram of soil has four cubic centimeters of water to give up its bacteria to. 

 Subsequent dilutions are based on the volume of the aliquot of the lower 

 dilution taken. The soil that makes up part of the aliquot is considered as 

 water. So little of the field soil is soluble and such a small part of it really 

 gets through to the high bacterial dilutions that the soil in the aliquot is 

 usually ignored. 



Basis of Dilutions 



The reason that the dilutions are based on 1-4 and not on the 1-2 or 1-10 

 is that it has been found that on the 1-4 basis the best plates from average 

 soil are secured from either the 1-40,000 or the 1-400,000 bacterial dilutions 

 of the bacteria in the soil. 100 colonies on plates from the 1-40,000 bacterial 

 dilution mean 4 million bacteria per gram of field soil while 100 (colonies on 

 plates from the 1-400,000 dilution mean 40 milhon bacteria per gram of field 

 soil taken. This allows a larger variation in the bacterial content to be handled 

 more satisfactorily with the same number of dilutions and technic than can 



