240 CLEAN MILK 



High Humidity. — All bacteria in the active state require the 

 presence of a great proportion of water : water dissolves their foods., 

 dilutes their excreta, and acts as a vehicle for their movements; but 

 above all, water is necessary for the passage of foqd materials in- 

 ward through the cell wall and for the passage of excreta outward. 

 Water forms 90% or more of the ordinary laboratory media. To 

 prevent concentration of the media by evaporation in the incubator, 

 the tubes may be sealed, or, more conveniently, water kept in bowls, 

 etc., in the incubator. 



Darkness. — Unlike most other living things, animal and vege- 

 table, direct sunlight is fatal to bacteria in a short time, diffuse sun- 

 light in a longer time ; absolute darkness favoring them most. The 

 incubator, and, in fact, all containers used for bacterial cultures,, 

 should be absolutely dark. 



Identification. — The recognition of a given bacterium as be- 

 longing to a certain species involves : 



1st. Isolation — by plating and selecting colonies for reinocu- 

 lation (see p. 256). ( 



2nd. Working out the characteristics of the isolated cultures — 

 preferably following the form suggested by the Soc. Am. Bact, 

 (see p. 282). 



3rd. The comparison of the features thus determined with 

 those already described by other workers, as in Chester's Manual, 

 or similar tabulated descriptions. Certain well known bacteria 

 (B. coli, B. tuberculosis, streptococci, etc.) can be determined by- 

 certain special tests without doing the elaborate work in full above 

 outlined. These tests are described in their proper places. 



