78 METHODS OF CULTIVATION OF BACTERIA. 



much in design. Sometimes a mechanism devised in 

 Koch's laboratory is affixed, which automatically turns off 

 _ the gas if the light be accidentally ex- 



. tinguished. 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. 32. 



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 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 per- 

 pendicular 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 

 ^eichert s QUt b e i ow t o a comparatively 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 r, 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. 



