94 BACTERIOLOGY. 



accidents should the flame become extinguished at a time 

 when no one was near. The gas-cock is provided with 

 a long arm which is weighted and which, when the gas 

 is turned on and burning, rests upon the end of a metal 

 spiral which is heated by the flame. If by draughts or 

 any other accident the flame becomes extinguished the 

 metal spiral cools, and in cooling contracts and allows 

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

 the gas supply is turned oiF. 



Theemo-eegdlatoes. — The regulation and mainte- 

 nance of the proper temperature within the incubator 

 is accomplished by the employment of an automatic 

 thermo-regulator. 



The 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 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 involved is seen in Fig. 18. 



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

 ing branch tube 6, and rubber stoopper /, 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. 



When ready for use the cylinder e is filled with fluid, 

 either mercury, calcium chloride solution, alcohol and 

 ether, or a number of other substances depending upon 

 the temperature to which it is to be subjected, up to about 



