446 



MERCURIAL GAS-PUMP. 



[Book ii. 



can be poured, and a metal casing round the tap d, which may also 

 be filled with water, similarly prevents the possibility of any leak- 

 age here. 



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

 tion shewn in the figure and raising B, the bulb A may be filled with 



O^ 



^ 



Fig. 88. Diagram of Alvergniat's Pump. 



mercury up to the top, the contained air being 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, ft being closed, a partial vacuum is 

 established in C and D. By means of the tap d the air in A may be 

 cut off 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 

 J. 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 facili- 





