288 PEOCEEDIXGS OF THE IXDIAXA ACADEMY OF SCIENCE. 



be handled on the 1-2. 1-200. 1-2,000, etc., or the 1-10. 1-100, 1-1,000, etc. 

 systems of dilution. Calculations are not hard as the number of colonies 

 divided by 25 or 2.5, according to the dilution, gives millions of bacteria per 

 gram of field soil. 



Number of Bacterial Dilutions Necessary 



Results obtained where the bacterial content is low should be comparable 

 with results obtained where the bacterial content is high, therefore a uniform 

 system for making dilutions is advisable. Suppose the 1-40 bacterial dilution 

 of a gravel should contain few enough bacteria to yield good plates but that 

 it was necessary- to make a 1-40.000 bacterial dilution of a sandy soil in order 

 to get as good plates. A 1-40 dilution of the sandy soil should be made in 

 the series of dilutions to have the results comparable for maximum errors 

 occur in the first bacterial dilution made. 



Representative aliquots are necessary if results are to approach accuracy. 

 It takes larger aliquots of some solutions and mixtures to have the aliquots 

 represent the solution or material under investigation than it does of others. 

 Using a pipette graduated to .005 of a cubic centimeter, a .05 cc aliquot of 

 a clear, dilute sodium chloride solution would be represtmtative of the 

 sodium chloride present in tlie solution. A .05 cc aliquot of a turbid solution 

 would not be representative even if taken with the same pipette, for the 

 suspended material would interfere with the composition of the small aliquot 

 and affect the accuracy with which the pipette could lie used. In mixtures 

 of soil and water the soil particles vary in size, in shape, and in specific 

 gravity. An aliquot of a soil and water mixture must be rather large to be 

 representative (at all) of the mixture. A soil and water mixture is not homo- 

 geneous for the following reasons: 



1. It is not a solution. 



2. Soil is heavier than water and the i)articles settle out, even when 



the mixture is in motion becau,se of differences in size, in shape 

 and in specific gravity. 

 Errors that occur in aliquoting a soil and water mixture are: 



1. Some of the material in suspensitm is taken as part of the aliquot 



when an aliquot is drawn from the mo^'ing mixture. 



2. Solid material drawn as part of the aliquot clings to the walls of 



the pipettes, and they do not drain accuratelj^ as a result. 



3. Bacteria are in, or on, the soil particles besides being in suspension 



so it is practically certain that not all colonies are broken up 

 bj' the first shakings. 



The errors enumerated above can not be entirely eliminated but are cut 

 down when the size of the aliquot taken is increased. As a rule, in every 

 series of bacterial dilutions there are at least two dilutions from which plat- 

 ings are made. One of these dilutions is ten times another, thus, for uni- 



