148 



BACTERIOLOGY. 



as ether, alcohol, and calcium chloride solution, which 

 expand and contract rapidly and, regularly under slight 

 variations in temperature, are commonly employed ; 

 whereas for temperatures approaching the boiling-point 

 of water mercury is most frequently used. 



The temperature of the incubator is to be regulated, 

 then, by the use of some such form of apparatus as that 

 just described. It should be of sufficient delicacy to 

 prevent a fluctuation of more than 0.2° C. in the tem- 

 perature of the air within the chamber of the apparatus. 



Gas-peessuee Regulators. — A gas-pressure reg- 

 ulator is not rarely intervened between the gas-supply 



Fig. 33. 



Moitessier's gas-pressure regulator. 



and the thermo-regulator. This apparatus has for its 

 object the maintenance of a constant pressure of the 

 gas going to the thermo-regulator. There are several 



