86 METHODS OF CULTIVATION OF BACTERIA 



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

 double walls between which some fluid (water or glycerin and 

 w r ater) 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 sup- 

 plied 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 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. 38. 



It consists of a long tube / closed at the lower 

 end, open at the upper, and furnished with two 

 lateral tubes. The lower part is filled with mer- 

 cury up to a point above tlie level of the lower 

 lateral tube. The end of the latter is closed by 

 a brass cap through which a screw d passes, the 

 FIG. 38. Reichert's inner end of which lies free in the mercury. The 

 gas regulator. 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 perpendicular tube fits accurately 

 a bent tube g, drawn out below to a comparatively small open point c, 

 and having in its side a little above the point "a minute needle-hole 

 called the peephole or by-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 

 the screw d is as far out as it will go, then some of the mercury must be 

 removed. Similarly, if when the desired temperature is reached and the 

 screw d is as far in as it can go, the mercury does not reach c, some more 

 must be introduced. If the amount of gas which passes through the 



