AIR-PUMP AND CENTRIFUGE 53 



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

 poses, such as evaporating to dryness in vacuo, nitration 

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

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

 p. 54) 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 50 c.c. each is 

 required. It is essential to balance carefully the opposite 

 tubes in the centrifuge, otherwise much vibration ensues 

 which interferes with complete sedimentation and is very 

 detrimental to the machine. The maximum speed 

 should be not less than 3,000 revolutions per minute. 

 The starting should be gradual, more and more power 

 being " turned on " as the machine acquires velocity. 

 Many forms of centrifuges are*obtainable. 



Bell-jars with ground rims 'and one or two tubules are 



