THERMO-REG ULA TORS. 145 



the flame become extinguislied 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 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 ^vhicll 

 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 branch tube b, 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 distance above this 

 point with a capillary opening, g, in one of its sides. 



10 



