114 BACTERIOLOGY. 



the flame become extinguished at a time when no one 

 was near. The gas-cook 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 plate that is 

 connected with the free ends of two metal spirals which - 

 are fixed 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 

 plate 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-eegulatoes. — 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 

 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 involved is seen in Fig. 28. 



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

 cating branch tube h, 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. 



