124 BACTERIOLOGY 



draught of air or from any other cause. The automatic burner 

 invented by Koch is an ingenious, simple and effective device 

 (Fig. 52). The coils of metal seen on each side at the top of the 

 burner are so arranged that when they expand they turn the 

 disk below so as to support the arm coming from the stop-cock; 

 when they cool they turn the disk in the opposite direction, and 

 allow the arm to fall and cut off the gas. Some inconvenience 

 will at times arise from irregularities in the flow of gas from the 

 main supply-pipe. A properly constructed regulator should, 

 however, compensate perfectly for all ordinary variations in pres- 

 sure of artificial gas. Natural gas is commonly furnished at 

 much higher pressure and it is necessary to install apparatus 

 to reduce the pressure, a gas-pressure regulator, between the gas 

 main and the thermoregulator. Fluctuations of the temperature 

 within the incubator depend very largely, upon the external 

 temperature, especially if its outer walls are not well insulated. 

 The incubator should, therefore, be kept in a place free from 

 draughts of air, where the temperature is fairly constant. 



In large modern laboratories, the incubators are built in as 

 special insulated rooms, heated by a gas stove. A regulator of 

 large size is installed to control the supply of gas to the stove. 

 These incubator rooms are very satisfactory and provide quite a 

 range of constant temperature according to the height of shelves 

 from the floor. 



Culture-cubes which are being kept in the incubator are hkely 

 to become dry if their stay is prolonged. In such cases they 

 should be covered with rubber caps, tin-foil, seaHng-wax, paraflSn, 

 or some other device to prevent evaporation. If rubber caps 

 are used, they should be left in i-iooo bichloride of mercury 

 solution for an hour, and the cotton plugs should be singed in the 

 flame, before putting them on (Fig. 47). Some bacteriologists 

 prefer rubber stoppers, which may be boiled and stored in bi- 

 chloride of mercury solution. Cut the cotton plug even with the 

 edge of the tube; singe it in the flame; push it into the tube about 

 I cm., and insert the rubber stopper (Fig. 46). 



