STORING AND INCUBATION OF CULTURES. 



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 y 



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. Some- 

 times a mechanism devised in Koch's laboratory 

 is affixed, which automatically turns off the gas 

 if the light be accidentally extinguished. Be- 

 tween 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. 47. 



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 mercury up to a point above FIG. 47. Reichert's 

 the level of the lower lateral tube. The end of the latter is as re s ulator - 

 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 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, <:, 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 ^, and the flame goes down. The contents of the jacket cool, the mer- 

 cury 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 peephole is suf- 

 ficient still to raise the temperature of the chamber when c is closed by the 

 rise of the mercury, then the peephole is too large. Tube c must be unshipped 



