76 MICRO-ORGANISMS IN WATER 



gelatine-peptone. The whole is then gently agitated, 

 allowed to solidify, and incubated at 20-22 C., and 

 the resulting colonies counted and examined in the 

 usual way. Miquel uses about a dozen such flasks for 

 each water examination. The number of bacteria ori- 

 ginally present in the sample can then be calculated 

 from the number of colonies which make their appear- 

 ance in the gelatine-films of these flasks. Excepting in 

 the use of the cumbrous and otherwise inconvenient 

 conical flask in question, this so-called ' mixed process ' 

 of Miquel's does not differ in any single detail from the 

 ordinary method of plate-culture as commonly prac- 

 tised, for it should be pointed out that preliminary 

 dilution before plate-cultivation must invariably be re- 

 sorted to in the case of all waters which, like sewage,, 

 polluted streams, &c., are very rich in bacterial life. 

 Such preliminary dilution is best made with sterilised 

 natural water, and not with sterilised distilled water, 

 as the latter is liable to prejudicially affect some bac- 

 teria. 



NUMERICAL DETERMINATION OF BACTERIA IN WATER 

 BY GELATINE-CULTURES 1 



The method of pouring gelatine-plates has already 

 been given (see p. 30), and it only remains here to de- 

 scribe the process as applied to water examinations. 

 Gelatine-plates, small round covered dishes, or Esmarch- 

 tubes, may all be employed for this purpose. The fre- 

 quent presence of microbes in water which liquefy the 

 gelatine renders the Esmarch-tubes less serviceable than 



1 For the qualitative determination of the bacteria present in any 

 given water, besides examining the colonies on the gelatine-plates with ;i 

 low power under the microscope (see p. 35) and inoculating particular 

 colonies into gelatine-tubes &c., recourse must be had to the special 

 methods described on pp. 267, 276, when typhoid or cholera bacteria are 

 suspected of being present. 



