232 BACTERIOLOGY. 
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 quan- 
tity of gas to the burner through the tube D (Fig. 33). 
BAUSCH & LomMa._—__- 
Small incubator. 
Thermo-regulator. 
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 surrounding 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 
