THERM 0-REG ULA TORS. 139 



off the gas-supply and thus preventing accidents should 

 the flarne 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 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 main- 

 tenance of the proper temperature within the incubator 

 are accomplished by the employment of an automatic 

 thermo-regulator or thermostat. 



The common form of thermo-regulator used for this 

 purpose is constructed upon principles involving the 

 expansion and contraction of fluids under the influence 

 of heat and cold. By means of this expansion and con- 

 traction the amount of gas passing from the source of 

 supply to the burner may be either diminished or in- 

 creased 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 con- 

 sists of a glass cylinder, e, having a communicating 

 branch tube 6, and rubber stopper /, through which pro- 

 jects the bent tube a. The tube a is ground to a slant- 



