88 METHODS OF CULTIVATION OF BACTERIA 



genie organisms. For the purpose of maintaining a uniform 

 temperature incubators are used. These vary much in the 

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

 double walls between which some fluid 

 & (water or glycerin and water) is placed. 



""-- a This, 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 supplied 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. 



The gas enters at a and from b passes to the 

 burner. When the mercury in / expands to cut 



FIG. 38. Eeichert's off the gas at c sufficient passes by the bye-pass e 

 gas regulator. to keep the flame alight. There is an improved 



form with a large bulb filled with xylol attached 



at/. Changes in the bulb of the xylol are communicated to the mercury. 

 This instrument is very delicate and will be found to work well. 



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, as 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 

 of water from which evaporation may take place. With tubes 

 which will require to be long in the incubator, the plugs should 



