86 



MANUAL OF BACTERIOLOGY. 



gas is of such variable pressure as to be entirely useless. 

 Fluctuations of the temperature within the incubator depend 

 very largely upon the external temperature. Therefore, the 

 incubator should, as far as is practicable, be protected from 

 sudden draughts of cold air and should be kept in a room 

 having as equable a tempera ture as possible. In large 

 laboratories it is often found convenient to use the whole of 

 a small room as an incubator, heating it by a gas stove, to 

 which a gas regulator may be applied. 



Culture-tubes which are being kept in the incubator arc 

 likely to become dry if their stay is pro- 

 longed. In such cases they should be 

 covered with rubber caps, tin-foil, seal- 

 ing-wax, paraffin 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. 21.) The wi iter prefers rubber 

 stoppers, which may be boiled and stored 

 in bichloride of mercury solution. In 

 using stoppers in this way the cotton plug is cut even with the 

 mouth of the tube, the top singed in the flame, and the plug 

 shoved down into the tube for about i cm. The rubber 

 stopper is then inserted (Fig. 20). 



Low-temperature Incubator. — An incubator regulated for so- 

 called "room temperature" is very desirable for the cultivation 

 of bacteria upon gelatin and for the bacteriological analysis 

 of water. In our climate the temperature of the rooms of the 

 laboratory often reaches a point at which gelatin melts, and 

 for this reason in a low-temperature incubator provision has to 

 be made for cooling when the room temperature is too high as 

 well as for heating when it is too low. 



Fig. 26. Koch Auto- 

 matic Gas-burner. 



