160 BACTERIOLOGY 



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, horizontal disk that is connected with 

 the free ends of two metal spirals which are fixed by their 

 other ends in opposite directions on either side of the flame 

 and heated by it. If by draughts or any other accident 

 the flame becomes extinguished, the metal spirals cool, and 

 in cooling contract, twist the horizontal disk in the opposite 

 direction, and by thus removing the support allow the 

 weighted arm of the gas-cock to fall. By its falling the gas- 

 supply is turned off. 



Thermo-regulators. The regulation and maintenance of 

 the proper temperature within the incubator are accom- 

 plished by the employment of an automatic thermo-regulator 

 or thermostat. 



The common form of thermo-regulator used for this 

 purpose is constructed uj>on principles involving the expan- 

 sion and contraction of fluids under the influence of heat 

 and cold. By means of this expansion 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 is seen in Fig. 30. It consists of a 

 glass cylinder, e, having a communicating branch tube 6, 

 and rubber stopper /, through which projects the bent tube 

 a. The tube a is ground to a slanting point at the extremity 

 which projects into the tube e, and is provided a short dis- 



