128 BACTERIOLOGY. 



off the gas-supply and thus preventing accidents should 

 the flame become extinguished at a time when no one 

 is near. The gas-cock by which the gas is turned on 

 and off is provided with a long arm which is weighted, 

 and which, when the gas is turned on and burning, rests 

 upon an arm attached to the side of a revolving, hori- 

 zontal disk that is connected with the free ends of two 

 metal spirals which are fixed by their other ends in oppo- 

 site directions on either side of the flame and heated by 

 it. If by draughts or any other accident the flame be- 

 comes extinguished, the metal spirals cool, and in cool- 

 ing contract, twist the horizontal disk in the opposite 

 direction, and allow the weighted arm of the gas-cock 

 to fall. By its falling the gas-supply is turned off. 



Thermo-eegulators. — The regulation and main- 

 tenance of the proper temperature within the incubator 

 are accomplished by the employment of an automatic 

 thermo-regulator. 



The common form of thermo-regulator used for this 

 purpose is constructed upon principles involving the 

 expansion and contraction of fluid substances under 

 the influence of heat and cold. By means of this ex- 

 pansion and contraction the amount of gas passing 

 from the source of supply to the burner may be either 

 diminished or increased as the temperature of the 

 substance in which the regulator is placed either rises 

 or falls. 



The simplest form of thermo-regulator which serves 

 to illustrate the principles involved is seen in Fig. 32. 



It consists of a glass cylinder e, having a communi- 

 cating brancli tube 6, and rubber stopper /, through 

 which projects the bent tube a. The tube a is ground 

 to a slanting point at the extremity wiiich projects into 



