THERMO-REG ULA TORS. 123 



off the gas supply and thus preventing accidents should 

 the flame become extinguished at a time when no one 

 was 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 the side of a revolving, horizontal disc that is con- 

 nected 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 disc 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-REGULATORS. The regulation and mainte- 

 nance of the proper temperature within the incubator 

 is 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 expan- 

 sion and contraction the amount of gas passing from 

 the source of supply to the burner may be either dimin- 

 ished 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. 29. 



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

 cating 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 



