JTew For)}i of Vacuum Stopcock. 



79 



A reference to the figure, which shows a section of the 

 htirrel .r and the phig y of the stopcock in position, will make 

 its action clear without much description, The barrel is 

 inclined to the horizontal at an angle of about 45°, and, in 

 the position ot the plug figured, a through connexion exists 

 between the tubes A and B. When the plug is turned 

 through 180°, the separate portions of mercury »ii, in^, m-i 

 seal off B both from the atmosphere and from A. C is a 



trap to catch any excess of the mercury 7n^. It may be 

 worth while to note that the stopcock should be ground 

 fairly conical in shape, as, if there is a yacuum in A, the 

 plug is forced into the barrel by the whole atmospheric 

 pressure ; also, that there is a true mercury sealing only in 

 the direction from A to B, so that the apparatus into which 

 it is desired to preyent leakage should be connected to this 

 latter tube. 



To get some idea of the extent to which a stopcock of this 



