48 A MANUAL OF BACTERIOLOGY 



115 to 125 C. When sterilising media care should be 

 taken that the vessels are not too full, and that the auto- 

 clave is allowed to cool down below 100 C. before opening 

 the stopcock, or some of the contents may be lost by 

 violent ebullition. While the temperature is rising, the 

 stopcock should always be left open until steam is being 

 freely generated so that the air may be expelled. 



Air-pump. An exhaust pump is useful for many pur- 

 poses, such as evaporating to dryness in vacuo, filtration 

 through porous porcelain filters, etc. Any form will serve, 

 but of the more elaborate ones the Fleuss pump (Fig. 5, 

 p. 50) made by the Pulsometer Engineering Company is 

 one of the best. In using it care must be taken that no 

 fluid or moisture gains access to the barrel ; to avoid 

 this the connecting pipe may be intercepted with a vessel 

 containing strong sulphuric acid (D, Fig. 5), over the 

 surface of which the exhausted air has to pass. A double- 

 necked Woulfe's bottle is suitable for this, the inlet and 

 outlet tubes extending nearly down to, but not dipping 

 below, the surface of the sulphuric acid. 



For greasing the vessels, etc., to make air-tight joints, 

 beeswax dissolved in the Fleuss pump oil with the aid of 

 heat to a stiff paste is a good composition, or the resin 

 ointment of the Pharmacopoeia may be used. Another 

 good grease is made by melting together one part of black 

 rubber, one part of vaseline, and one- third part of paraffin 

 wax. 



Centrifuge. A small centrifuge holding two or four 

 10 c.c. tubes is a necessity in the laboratory. A form 

 driven by hand may be used, but one driven by water or 

 electricity is almost essential. If milk is examined, a 

 centrifuge driven by power and containing two or more 

 tubes having a capacity of not less than 70 c.c. each is 

 required. Many forms of centrifuges are supplied by 

 Messrs, Hearson, 



