84 METHODS OF CULTIVATION OF BACTERIA 



water vapour. Whenever the vacuum is produced the rubber- 

 tube should be at once disconnected from 5, the cock a being 

 shut. It is advisable when the apparatus is exhausted to cover 

 the vacuum chamber and the Wolffs bottle with wire guards 

 covered with strong cloth, in case, under the external pressure, 

 the glass vessels give way. The connecting and disconnecting 

 of rubber-tubing of sufficient thickness to withstand collapse 

 when exhausted is difficult. Ordinary stout rubber-tubing can 

 be used if through it there is passed a length of narrow flexible 

 metal-tubing, the ends of which project beyond the rubber-tubing 



Fig. 33. — Geryk air-pump for drying in vacuo. 



so as to enter the parts of the apparatus to which the latter is 

 fitted. 



The Storing and Incubation of Cultures. — Gelatin cultures 

 must be grown at a temperature below their melting-point, i.e., 

 for 10 per cent, gelatin, below 22° C. They are usually kept in 

 ordinary rooms or in a cool incubator at about 20° C. Agar, 

 and serum media are employed to grow bacteria at a higher 

 temperature, corresponding to that at which the organisms 

 grow best, usually 37° C. in the case of pathogenic organ- 

 isms. 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. 



