METHODS FOR MICROSCOPIC STUDY OF BACTERIA 245 
3. ■ The Incubation of Bacterial Cultures. —The ^rowtli of bacteria in 
artificial media is markedly iiiHuenced hy the temperature to which 
they are exposed. A majority of those organisms, parasitic upon or 
pathogenic for man, develop most luxuriantly at the temperature of 
the human body, 37° C. Exposure to temperatures but slightly 
above 37° C. leads to rapid death of these organisms, consequently 
Fig. ,34. — Incubator. (Park.) 
incubators must be available within which cultures may be safely 
exposed to a uniform and constant degree of heat equal to that of the 
human body. Gelatin cultures must be maintained at a temperature 
not exceeding 22° C. 
Incubators are single- or double-walled chambers of various sizes, 
heated directly by gas or electricity, or indirectly through a water 
jacket. The latter run more uniformly, because water receives and 
