52 LABORATORY GUIDE IN BACTERIOLOGY 



circulates. The outer surface is usually covered with asbestos 

 or linoleiun, 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 causing a drop in temperature by admitting 



Fig. 20 

 Theimostat or Incubator 



the cooler air. The outer door is covered with asbestos like 

 the walls. A thermometer reaches through the upper part, 

 provided with several air holes which permit free circulation 

 of air. The gas supply is shut ofi automatically, if it should 

 accidentally become extinguished. The necessary heat may also 



