THE CULTIVATION OF BACTERIA. 91 



from the main supply-pipe. Any incubator will vary a little 

 from such causes. In the experience of the writer, natural 

 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 temperature as possible. 



Culture-tubes which are being kept in the incubator are 

 likely to become dry if their stay is prolonged. In such 

 cases they should be covered with rubber caps, tin-foil, 

 sealing-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 put- 

 ting them on. (Fig. 21.) The writer prefers rubber 

 stoppers, which may be boiled and stored in bichloride 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. 20.) 



CULTIVATION OF ANAEROBIC BACTERIA. 



The cultivation of anaerobic bacteria is done best in a 

 medium containing i to 2 per cent, of dextrose. The tube 

 should contain a large quantity of the culture-medium. 

 Just before using, the medium should be boiled for a few 

 minutes. Inoculate the tube after cooling, but while the 

 medium is fluid. Anaerobes may be cultivated in the 

 closed arm of the fermentation-tube (see Fig. 46), but the 

 opening between the two arms of the tube must be small. 



Buchner's method for the cultivation of anaerobes: Into 

 a bottle or tube which can be tightly stoppered, pour 10 c.c. 



