A SIMPLE FORM OF SPRENGEL VACUUM PUMP. 141 



Figure 1, p. 140, is a sketch of the pump, in section, drawn 

 to scale, one eighth the full size. Though the dimensions 

 are not of importance in this instance, the diagram suffici- 

 ently shows those which have been found suitable. The 

 apparatus, which is all of glass, is fastened to a board 

 resting on a wooden base, which can be tilted by screws, 

 one of them being shown at S. B is the bulb in which the 

 mercury is boiled by means of the heat from the flame of 

 the gas ring G. From this the mercury vapour passes 

 upwards and is condensed in the Liebig's condenser C. The 

 drops of condensed mercury roll down the inner tube of the 

 condenser into the fall tube T, there carrying the gas from 

 the vessel to be exhausted before them, the gas escaping 

 through the mercury seal at A into the pipe D, which is 

 connected with an auxiliary pump, and the mercury flows 

 back into the bulb B by the tube R. 



In such a pump the mercury passing through the fall tube 

 is always clean; the mercury simply circulates round the 

 apparatus, touching only glass, without passing through 

 any taps, and is never in contact with air. The auxiliary 

 pump should be easily worked and capable of exhausting 

 rapidly to a pressure of a millimetre or less of mercury, a 

 Geryk oil pump being admirable for the purpose. If a less 

 efficient pump is used, the seal of the fall tube becomes 

 longer, entailing compression of the gas bubbles and intro- 

 ducing the risk of their not being carried right through by 

 the falling drops. 



The vessel to be exhausted is fused to the tube of the 

 drying flask shown at the right hand top corner of the 

 sketch, the flask and gauge F being attached by a glass 

 tube to the fall tube T. The connecting tube is compara- 

 tively long to minimise the danger of mercury vapour 

 passing over into the vessel to be exhausted. The small 

 bore tube T, similar to those used for the fall tubes in any 



