90 SCIENTIFIC APPARATUS. 



SprengePs pump is the type of the other set. The working 

 part is a vertical glass tube longer than the height of the barometer, 

 and so narrow that a small portion of mercury placed in it is com- 

 pelled by its surface-tension to fill the whole section of the tube. 

 The mercury is introduced into this tube from a funnel at the top 

 through a small India-rubber tube regulated by a pinch-cock, so that 

 die mercury falls in small detached portions, each of which drives 

 before it any air which may be in the tube till it escapes into a 

 mercury-trough, into which the bottom of the tube dips. 



The vessel to be exhausted is connected to a tube which enters 

 at the side of the vertical tube near the top. Any air or other 

 gas which may be in the vessel expands into the vacuum left in 

 the vertical tube between successive portions of the falling mercury, 

 and is driven down the tube by the next portion of mercury into 

 the mercury-trough, where it may be collected. 



As long as the rarefaction of the air is not very great the quantity 

 of air which is included between successive portions of mercury is 

 sufficient to act as a sort of buffer, but as the rarefaction increases 

 the portions of mercury come together more abruptly, and produce 

 a sound which becomes sharper as the vacuum becomes more 

 perfect. 



After the mercury-pump has been in action for some hours the 

 quantity of matter remaining in the vessel is very small. If the 

 tubes have been joined by means of caoutchouc connections there 

 is a trace of gaseous matter emitted by the caoutchouc. It is there- 

 fore necessary, when a very perfect vacuum is desired, to make 

 all the joints " hermetical " by fusing the glass. Still, however, 

 there remains a trace of matter. The vapour of mercury is, of 

 course, present, and the sides of the glass vessel retain water very 

 strongly, and part with it very slowly, when all other matter is 

 removed. 



By passing a little strong sulphuric acid through the pump along 

 with the mercury, vapours both of mercury and of water may be 

 in great measure removed. 



