QUANTITATIVE EXAMINATION 855 



organism originally present in the water : by counting the colonies the 

 number of organisms present in the volume of water sown can thus be deter- 

 mined. The flask must be examined daily ; the best way is to invert it ; 

 the colonies are then visible in the transparent medium through the bottom 

 of the flask, and can be readily counted. 



Thus, if on the third day, twelve colonies are visible a note should be 

 made. 



3rd day. 5 August. 12 colonies. 



To avoid counting the same colonies twice, mark their position with a pen 

 and ink on the base of the flask as they are counted. 



If next day in addition to the twelve colonies marked eight more have 

 appeared the note will read 



3rd day. 5 August. 12 colonies 



4th day. 6 August. 20 colonies 



and so on. 



The enumeration is ordinarily completed by the fifteenth to the twentieth 

 day, after which no new colonies are likely to appear. Suppose then that the 

 result is 



20th day. 22 August. 64 colonies 



the number of aerobic organisms in a cubic centimetre of the water will be 

 ascertained by multiplying 64 by 100. The water therefore is said to contain 

 6400 organisms per c.c. 



It is as well to sow several flasks with every sample of water, which may be num- 

 bered 1, 2, 3 and so on ; the mean of the various results will be nearer the truth 

 than the result of a single determination is likely to be : and if as often happens 

 the number of organisms in the flasks varies but little, the accuracy of the technique 

 is demonstrated and the results are of greater value. 



In the case illustrated it is assumed that no liquefaction of the gelatin has 

 occurred to interfere with the enumeration. Unfortunately, however, this 

 is not the usual experience in practice , it is much more common to find that 

 the water contains a high percentage of " liquefying " organisms so that 

 after the first few days further enumeration is rendered difficult and finally 

 impossible. In such cases the count should be continued until the plate is 

 entirely liquefied and then the date of liquefaction noted. Thus, for instance, 

 2nd day, - 26 colonies 



3rd 59 



4th - 102 



5th - Plate entirely liquefied 



the results of the analysis should be recorded thus 



10,200 (102 x 100) aerobic organisms per cubic centimetre. This number is 

 much beloiv the real total, liquefaction of the gelatin having terminated the count 

 on the fifth day. 



The statement of the result must be qualified in this way whenever liquefac- 

 tion occurs before about the tenth day. 



Note. Many waters contain moulds as well as bacteria. The moulds 

 develop on the plates and must be separately enumerated. Thus, for instance, 

 a sample of water might contain 



1256 aerobic bacteria and 300 moulds per cubic centimetre. 



B. Method of enumeration using a -V c - c - pipette. The dilution method 

 is not only rather tedious but in inexpert hands affords opportunities of con- 

 tamination. The method now about to be described is therefore often 

 preferred. 



The pipettes made by Alvergniat should be used : they are very carefully 

 calibrated and give about 50 drops to the cubic centimetre. Pipettes giving 



