556 



MERCURIAL GAS-PUMP. 



[BOOK ii. 



The pump is used as follows. By placing the tap d in the position 

 shewn in the figure and raising B, the bulb A may be tilled with mercury 



FIG. 74. DIAGRAM OF ALVERGXIAT'S PUMP. 



up to the top, the contained air 1 icing expelled through the upper end 

 of b. By a slight turn of the tap all connection between A and 

 either the tube g or the upper part of b may be cut off, and on lowering 

 B a vacuum is established in the bulb A and part of the tube a. A 

 may now be connected by the tap d with the tube g, and hence with C 

 and D, and, h being closed, a partial vacuum is established in C and D. 

 By means of the tap c/ the air in A may be cut oft' from g, and on 

 raising B and placing the plug of d as shewn in the figure this air may 

 be expelled through the upper end of b. By slightly turning d and 

 lowering B a vacuum is again established in A, and as before a further 

 portion of air in C and D may be allowed to pass over into A and the 

 vacuum in D and C increased. In this way all the air in D can 

 be extracted, the final stages being facilitated by the admission of a 

 little water into D, the last traces of air being driven over into A by 

 the rush of vapour from the water. A known volume of blood having 

 been collected over mercury in a small tube is now allowed to enter 1) 

 through the tap h and yields up its gases to the vacuum. A repetition 



