INCUBATORS OR THERMOSTATS 157 



with double walls and double doors, and the space between the two 

 walls is filled with water, a poor conductor of heat, which tends to 

 keep a fairly steady, uniform temperature in the apparatus. The 

 source of heat for a bacterial incubator is now, as a rule, illuminating 

 gas, though some have an arrangement which will permit the use of 

 a coal-oil lamp in places where gas is not accessible. The most modern 

 incubators use electrical appliances as a source of heat. 



Thermoregulator. In order to keep a fairly uniform temperature 

 in a bacterial incubator it should be so located that it is protected 

 against very sudden changes of temperature and must be provided 

 with a thermoregulator. The latter is a device or apparatus which 

 will automatically regulate the supply of heat. Since incubators 



FIG. 79 



Thermostat, or incubator. 



generally receive their source of heaj; from illuminating gas, most 

 thermoregulators are designed to control the gas supply, which goes 

 to the gas flame burning underneath the thermostat. The temper- 

 ature in the incubator is controlled by a thermometer projecting 

 through the upper double wall into the air space or chamber where 

 the cultures are kept. The flame generally used under a thermostat 

 is a so-called micro-gas lamp, a small burner which furnishes a small, 

 narrow but high flame protected by a mica cylinder. Frequently a 

 Koch-Pfeil safety lamp is used. This is so constructed that it will 

 automatically shut off the gas supply if the flame should be blown 

 out by a draft of air or for some other reason. However, if the thermo- 

 stat is in a protected place this danger is very remote. A greater 

 danger comes from leaks in the connecting rubber hose, and this must, 



