MOLECULAR PHYSICS. 89 



Hauksbee it remained in use until the introduction of Smeaton's 

 pump, which, however, has not wholly superseded it."* 



The history of the air-pump after this time relates chiefly to the 

 contrivances for insuring the working of the valves when the 

 pressure of the remaining air is no longer sufficient to effect it, and 

 to methods of rendering the working parts air-tight without intro- 

 ducing 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 tjie receiver. It has 

 been shown 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 in it, and then to flow back leaving 

 the chamber empty. 



* Thomas Young's Lectures on Nat. Phil. (1807). Lecture xxx. 

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



