STORING AND INCUBATION OF CULTURES 79 



stand which rests on the brass plate. The edge of the bell-jar 

 is well luted with unguentum resinae and placed in position and 

 the chamber exhausted. In a few hours, if, as is always advis- 

 able, each dish have contained only a thin layer of fluid, the 

 drying will be complete. The vacuum is then broken by 

 admitting air very slowly through a bye-pass c, and the bell-jar 

 is removed. In such an apparatus it is always advisable, as is 

 shown in the figure, to have interposed between the pump and 

 the vacuum chamber a Wolffs bottle containing sulphuric acid. 

 This protects the oil of the pump from contamination with 



FIG. 39. Geryk air-pump for drying in vacuo. 



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 Wolffs bottle with wire guards 

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

 the glass vessels give way. 



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 usually employed to grow 

 bacteria at a higher temperature, corresponding to that at which 



