THE AIR-PUMP 27 3 



ie projecting part of the screw, or it has a wide open- 

 ig, the edge of which is ground flat, and when 

 reased with lard and placed upon the plate, closes 

 r-tight. The receivers used for most experiments 

 re wide-mouthed glass vessels with stout walls. The 

 -hole air-pump is fixed to the table by a strong 

 jrew-clamp. 



Suppose now that a receiver has been placed upon 

 .ie plate, and is to be exhausted. First, the stop-cock 



placed in the position I. ; the receiver is now con- 

 ected with the barrel, and by means of the tube a 

 ! ith the external air. The handle is then turned so as 

 j> move the piston to the extremity of the barrel, as far 

 * it can possibly go, so that no air may be left in the barrel 

 Btween the piston and the stop-cock, which is now 

 rought into position II., which stops the communica- 

 on of the receiver and barrel with the external air, 

 it not between the receiver and the barrel. If now 

 ie handle is turned in the opposite direction, the 

 ston moves away from the stop-cock, and leaves 

 ;i empty space, which, however, is immediately 

 iled, because the air in the receiver expands ; and if 

 1 e volume of the barrel were equal to that of the re- 

 <jiver, the air in the latter would expand to double its 

 j-evious volume, while its density would decrease by 



< e half. If the volume of the barrel were greater 

 tan that of the receiver, the increase in volume and 

 lie consequent diminution of density would be propor- 

 tmally greater ; on the other hand, if the receiver is 

 i eater than the barrel, the increase in volume and 



< ninution of density are less. When the piston has 

 r ached the end of the cylinder, the stop-cock is 



