276 THE SIPHON-GAUGE 



the air can be carried to a very high degree, although 

 in none of them a perfect vacuum is attainable. 



An air-pump is a rather expensive apparatus, and the student 

 should prefer to forego the advantage of possessing one rather than 

 purchase a cheap air-pump, which is certain to be useless. 



To preserve an air-pump in a good serviceable state, great care is 

 required. Dust should never be allowed to get inside the barrel, 

 for it would spoil it ; the pump should therefore always be kept in 

 a closed box, when not in use. In cold weather it should be 

 allowed to stand in the warm room for a time, so as to acquire the 

 warmer temperature ; if the piston is moved before, the pump is 

 apt to be damaged. The piston should always be well greased 

 with lard, which must be free from salt ; oil makes the whole sticky, 

 and salted lard attacks the metal. The stop-cock should move 

 smoothly, but not too easily ; if it is too loose, it may be somewhat 

 tightened by cautiously turning the screw s (fig. 187 I.) ; but if it is 

 too tight, it should be taken out and greased with tallow, care 

 being taken that none of it gets into the bores, and the stop- cock, 

 and the inside of the aperture in which it rests, very cautiously 

 cleaned. Special precautions with reference to particular expert 

 ments are given further on. Great care should be taken not to 

 let any mercury get inside the pump, for in that case the whole 

 would have to be taken to pieces, every part thoroughly cleaned, 

 fresh lard put on, and the parts again put together, an operation 

 which is rather difficult to an unpractised hand ; the necessity for 

 it should therefore be avoided by proper care. 



In a receiver sufficiently tall to allow of a barometer 

 being placed under it, the decrease of pressure mani- 

 fests itself immediately as the exhaustion proceeds ; 

 at every stroke of the piston the mercury descends in 

 the tube. Usually, however, the pressure in the re- 

 ceiver is measured by means of the siphon-c/auge, 

 that is, a short siphon-barometer, varying in height 

 from 6 to 20 cm , which may be conveniently placed 

 under a common receiver. If we suppose a short bent 

 tube with two legs of about equal length, one of which 

 is closed and filled with mercury, while the other is only 



