STORING AND INCUBATION OF CULTURES. 89 



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 /; to the burner. 

 When the thermometer in the interior of the 

 chamber indicates that the desired temperature v 



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 alterna- 

 tion 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 sufficient 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 and e plastered over 

 with sealing-wax, which is pricked, while still 

 soft, with a very fine needle. The gas flame, 

 when only the peephole is supplying gas, ought 

 to be sufficiently large not to be blown out by 

 small currents of air. If the pressure of gas 

 supplied to a regulator varies much in the 24 hours a pressure regulator 

 ought to be interposed between the gas tap and the instrument. 

 Several varieties of these can be obtained. In all cases g ought to be 

 fixed to b with a turn of wire. 



FlG. 36. Reichert's 

 gas regulator. 



The varieties of incubators are, as we have said, 

 numerous. The most complicated and expensive are 

 made by German manufacturers. Many of these are 

 unsatisfactory. They easily get out of order and are 

 difficult to repair. We have found those of Hearson of 

 London extremely good, and in proportion to their size 

 much cheaper than the German articles. They are fitted 

 with an admirable regulator. It is preferable in using an 

 incubator to connect the regulator with the gas supply and 

 with the Bunsen by flexible metal tubing. It is necessary 



