80 METHODS OF CULTIVATION OF BACTERIA 



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 glycerin and 

 water) is placed, which, when raised to a certain temperature, 

 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 

 gr design. Sometimes a mechanism devised in 



...- : ---.... Koch's laboratory is affixed, which auto- 



a matically turns off the gas if the light be 

 accidentally extinguished. Between the tap 

 supplying the gas, and the burner, is inter- 

 posed a gas regulator. Such regulators 

 vary in design, but for ordinary chambers 

 which require to be kept at a constant tem- 

 perature, Reichert's is as good and simple 

 as any and is not expensive. It is shown 

 in Fig. 40. 



It consists of a long tube /closed at the lower 

 end, open at the upper, and furnished ^ith 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 

 brass 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 



J 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 perpendicular tube fits accurately 

 FIG. 40. Reichert's a bent tube g, drawn out below to a comparatively 

 gas regulator. small open point c, 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 distance below the lower open end 

 of tube c. The burner is now lit. The gas passes in at a through c 

 and e and out at b to the burner. When the thermometer in the 

 interior of the chamber indicates that the desired temperature has been 

 reached, the screw d is turned till the mercury reaches the end of the 

 tube c. Gas can only now pass through the peephole e, and the flame 

 goes down. The contents of the jacket cool, the mercury contracts off 

 the end of tube c, and the flame rises. This alternation going on, the 

 temperature of the chamber is kept very nearly constant. If the mercury 

 cuts off the gas supply before the desired temperature is reached, and 



