56 LABORATORY GUIDE IN BACTERIOLOGY 



circulates. The outer surface is usually covered with asbestos 

 or linoleum, so as to hold the heat. The thermostat is provided 

 with two doors, the inner one of glass so as to enable the observer 

 to look inside without opening it, the other one covered like the 

 walls. A thermometer reaches through the upper part, and sev- 

 eral air-holes are provided in it, which permit free circulation 

 of air. The heat is controlled by Bunsen burners with self- 

 regulating devices. The gas supply is shut off automatically, if 

 it should accidentally become extinguished. For class use large 

 incubators are constructed on the same principles, with a number 

 of separate compartments. 



