STORING AND INCUBATION OF CULTURES 85 



This protects the oil of the pump from contamination with 

 water vapour. Whenever the vacuum is produced, the rubber- 

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

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

 the vacuum chamber and the Wolff's 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 7 flexible 



FIG. 37. Geryk air-pump for drying in varno. 



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

 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, which vary, of course, in temperature at different 

 times, but which have usually a range of from about 12 C. to 

 18 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 patho- 

 genic organisms. For the purpose of maintaining a uniform 

 temperature incubators are used. These vary much in the 



