INSTRUMENTS CONNECTED WITH FLUIDS. 525 



the working parts air-tight without introducing substances the vapour of which 

 would continue to fill the otherwise empty space. 



There is one form of air-pump, however, which we must notice, as in it 

 all packing and lubricating substances are dispensed with. This is the air-pump 

 constructed by M. Deleuil, of Paris*, in which the pistons are solid cylinders 

 of considerable length, and are not made to fit tightly in the barrels of the 

 pump. No grease or lubricating substance is used, and the pistons work easily 

 and smoothly in. the barrels. The space between the piston and the barrel 

 contains air, but the internal friction of the air in this narrow space is so great 

 that the rate at which it leaks into the exhausted part of the barrel is not 

 comparable with the rate at which the pump is exhausting the air from the 

 receiver. It has been shewn by the present writer that the internal friction 

 of air is not diminished even when its density is greatly reduced. It is for 

 this reason that this pump works satisfactorily up to a very considerable degree 

 of exhaustion. 



Pumps of the type already described are still used for the rapid exhaustion 

 of large vessels, but since the physical properties of extremely rarefied gases 

 have become the object of scientific research, the original method of Torricelli 

 has been revived under various forms. 



Thus we have one set of mercury-pumps in which the mercury is alternately 

 made to fill a certain chamber completely and to drive out whatever gas may 

 be hi it, and then to flow back leaving the chamber empty. 



Sprengel's pump is the type of the other set. The working part is a vertical 

 glass tube longer than the height of the barometer, and so narrow that a 

 small portion of mercury placed in it is compelled by its surface-tension to fill 

 the whole section of the tube. The mercury is introduced into this tube from 

 a funnel at the top through a small India-rubber tube regulated by a pinch- 

 cock, so that the mercury falls in small detached portions, each of which drives 

 before it any air which may be in the tube till it escapes into a mercury- 

 trough, into which the bottom of the tube dips. 



The vessel to be exhausted is connected to a tube which enters at the 

 side of the vertical tube near the top. Any air or other gas which may be 

 in the vessel expands into the vacuum left in the vertical tube between suc- 

 cessive portions of the falling mercury, and is driven down the tube by the 

 next portion of mercury into the mercury-trough, where it may be collected. 

 * Comptes Rendus, t. Ix. p. 571. Carl's Repertorium. 



