88 METHODS OF CULTIVATION OF BACTERIA. 



The Storing and Incubation of Cultures. Gelatine 

 cultures must be grown at a temperature below their 

 melting-point, i.e. for 10 per cent gelatine, below 22 C. 

 They are usually kept in ordinary rooms, which vary, of 

 course, in temperature at different times, but which have 

 usually a range of from about 12 C. to 18 C. Agar and 

 serum media are usually employed to grow bacteria at a 

 higher temperature, corresponding to that at which the 

 organisms grow best, usually 37 C. in the case of pathogenic 

 organisms. For the purpose of maintaining a uniform 

 temperature incubators are used. These vary much in the 

 details of their structure, but all consist of a chamber with 

 double walls between which some fluid (water or glycerine 

 and water) is placed, which, when raised to a certain tem- 

 perature, ensures a fairly constant distribution of the heat 

 round the chamber. The latter is also furnished with 

 double doors, the inner being usually of glass. Heat is 

 supplied from a burner fixed below. These burners vary 

 much in design. Sometimes a mechanism devised in 

 Koch's laboratory is affixed, which automatically turns off 

 the gas if the light be accidentally extinguished. Between 

 the tap supplying the gas, and the burner, is interposed a 

 gas regulator. Such regulators vary enormously in design, 

 but for ordinary chambers which require to be kept at a 

 constant temperature, Reichert's is as good and simple as 

 any and is not expensive. It is shown in Fig. 36. 



It consists of a long tube f closed at the lower end, open at the 

 upper, and furnished with two lateral tubes. The lower part is filled 

 with mercury up to a point above the level of the lower lateral tube. 

 The end of the latter is closed by a j^rass cap through which a screw d 

 passes, the inner end of which lies free in the mercury. The height 

 of the latter in the perpendicular tube can thus be varied by increasing 

 or decreasing the capacity of the lateral tube by turning the screw a 

 few turns out of or into it. Into the upper open end of the perpendi- 

 cular tube fits accurately a bent tube, g\ drawn out below to a com- 

 paratively small open point, f, and having in its side a little above the 

 point a minute needle-hole called the peephole or bye-pass e. To fix 

 the apparatus the long mercury bulb is placed in the jacket of the 

 chamber to be controlled, tube a is connected to gas supply, tube b 

 with the burner. The upper level of the mercury should be some 



