82 METHODS OF CULTIVATION OF BACTERIA. 



must always be water vapour present. An air-pump is there- 

 fore to be employed. Here we have found the Geryk pump 

 most efficient, and it has this further adyantage, that its 

 internal parts are lubricated with an oil of very low vapour 

 density so that almost a perfect vacuum is obtainable. The 

 apparatus is shown in Fig. 46. The vacuum chamber consists, 

 of a bell-jar set on a brass plate. A perforation in the centre 

 of the latter leads into the pipe a, which can be connected by 

 strong walled rubber tubing with the air-pump, and which 

 can be cut off from the latter by a stopcock b. In using 

 the apparatus the substance to be dried is poured out in flat 

 dishes (one-half of a Petri dish does very well), and these are 

 stacked alternately with similar dishes of strong sulphuric acid on 

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

 is well luted with unguentum resinse 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 ad- 

 mitting 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 Wolff's bottle containing sulphuric acid. 

 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 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 

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

 organisms. For the purpose of maintaining a uniform tem- 

 perature incubators are used. These vary much in the details 

 of their structure, but all consist of a chamber with double walls 



