92 CHEMICAL DISCOVERY AND INVENTION 



of liquids. For this particular purpose it is not necessary, nor 

 is it desirable, to arrange for a very great reduction of atmo- 

 spheric pressure beneath the filtering surface, which is almost 

 always formed of paper, the sides of which are supported by the 

 funnel, while the point of the cone is unprotected, or strengthened 

 only by a small cone of gauze or of toughened paper. The water- 

 pumps used for this purpose are supplied with high pressure 

 water, and their action is very similar to that of the steam 

 injector. 



Two forms of these pumps are shown in the preceding figures 

 a and 6. In both a jet of water under pressure escapes from the 

 downward-pointed nozzle, and is discharged into the open end 

 of the tube below, carrying with it air drawn from the space to 

 be exhausted by means of the side tube. The water is usually 

 carried into the sink by means of a piece of flexible tubing 

 attached at the bottom of the apparatus. 



The other form of water pump does not depend on the pressure 

 produced by a head of water, but on another principle which 

 requires the pump to be situated at a height a little greater than 

 30 feet above the surface of the earth or the well into which 

 water falling down a vertical pipe is discharged. The apparatus 

 was first described by Dr. Hermann Sprengel just fifty years 

 ago. If mercury is the liquid used, then the fall pipe requires to 

 be a little over the ordinary height of a barometer, or some 33 

 or 34 inches, and in this form the Sprengel pump was the parent 

 of most of the more complicated instruments devised later for 

 the purpose of removing air from an enclosed space. These 

 mercurial pumps have played a large part in the modern re- 

 searches on the gaseous state. The principle is so simple and 

 yet so very important that a description of Sprengel's pump, in 

 the inventor's words, 1 may be introduced here, as it will enable the 

 reader to understand immediately its more recent modifications. 

 The construction and use of the water pump also becomes obvious. 



" C d is a glass tube longer than a barometer, open at both 

 ends, and in which mercury is allowed to fall down, supplied by 

 the funnel A with which the tube is connected at C. The lower 

 end d of this tube dips into a small glass bulb B, into which it 

 is fixed by means of a cork. This glass bulb has a spout at its 

 side, situated a few millimetres higher than the lower end of the 

 tube C d. The first portions of mercury which run down will 



1 Journal Chem. Soc. t Vol. 18, p. 10 (1865). 



