64 



PRINCIPLES OF BACTERIOLOGY 



Apparatus for Obtaining a Suitable Temperature for the Growth of Bac- 

 teria. INCUBATORS. In order to have a constant and proper temper- 

 ature for the growth of bacteria, forms of apparatus called incubators 

 have been devised (Fig. 48). These consist, in their simplest form, of 

 an inner air chamber surrounded by a double copper wall containing 

 water. The apparatus externally is lined with asbestos, to prevent radia- 

 tion. It is supplied with doors and with openings for thermometers 

 and a thermoregulator. The thermoregulators are of various kinds; 

 those in' most use depend upon the expansion or contraction of the 

 fluid in the bulb A (Fig. 49), which rests within the water-jacket, to 

 lessen or increase the space between the surface of the mercury B 

 and the inner tube D, thus allowing of the passage of a greater or less 



FIG. 48 



FIG. 49 



Small incubator. 



Thermoregulator. 



quantitity of gas to the burner through the tube D. Other, forms 

 depend upon the contraction or expansion of metal, or the use of the 

 electric current to control the flow of the gas. 



The temperature in the air chamber is kept above that of the sur- 

 rounding air by means of a gas flame regulated as above described, 

 or, when that cannot be obtained, a lamp. 



The temperature is reduced by passing a stream of cool water through 

 the water chamber, which is itself regulated. When very accurate 

 investigations are to be made a gas-pressure regulator is added to the 

 thermoregulator. Incubators are also both warmed and regulated 

 by electricity. 



In emergencies a culture may be developed at the blood tempera- 

 ture by placing it in water contained in a small vessel, which itself 

 is contained in a larger vessel, also filled. By adding a little hot water 



