METHODS USED IN CULTIVATION OF BACTERIA 



193 



is poured into the cylinder to the depth of about two inches. An 

 inlet pipe conducts gas into the open space between the top of the 

 glycerin and the bell. From, the top of the bell is suspended a 

 conical piece of metal which hangs free in the outlet pipe. As the 

 gas pressure under the bell increases, this is raised and the opening 

 of the outlet pipe is gradually diminished by the cone. Thus the 

 relation between the pressure in the inlet pipe and the actual quan- 

 tity of gas passing through is equalized. A cup connected to the 

 top of the bell through the roof of the cylinder by a bar can be 

 filled with birdshot and the pressure against the gas can thus be 

 modified to conform with existing conditions. 



Colony Study. Cultures are usually incubated for from twelve 



FIG. 25. VARIATIONS IN THE CONFORMATION OP THE BORDERS OP BACTERIAL 



COLONIES. 



to forty-eight hours. Considerable aid to the recognition of species 

 is derived from the observation of both the speed of growth and 

 the appearance of the colonies. It is therefore necessary to proceed 

 in the study of developed colonies in a systematic way. The develop- 

 ment of colonies should be observed in all cases both upon gelatin 

 and upon agar. In forming any judgment about colonies, the acidity 

 or alkalinity, and the special constitution of the media should be 

 taken into account. The colonies are carefully examined with a 

 hand lens and with the low power (Leitz No. 2, Zeiss A A, Ocular 

 No. 2) of the microscope. The colonies should be observed as to 

 size, outline, transparency, texture, color, and elevation from the 

 surface of the media. Much information, also, can be obtained by 

 observing whether a colony appears dry, mucoid, or glistening, like 

 a drop of moisture. By a careful observation of these points, definite 



