STORING AND INCUBATION OF CULTURES 81 



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. 



FIG. 41. Hearson's incubator for use at 37 C. 



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 to see that 

 there is not too much evaporation from the surface of cultures 

 placed within incubators, otherwise they may quickly dry up. 

 It is thus advisable to raise the amount of water vapour in the 

 interior by having in the bottom of the incubator a flat dish full 

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