OF THE AIR-PUMP. 



215 



some, is done by suction. But to refute that erroneous 

 notion, let the air be pumped out of the receiver A B 

 and then all the quicksilver in the tube will fall down by 

 its own weight into the jar ; and cannot be again raised 

 one hair's breadth in the tube by working the syringe ; 

 which shews that suction had no hand in raising the 

 quicksilver ; and, to prove that it is done by pressure 

 let the air into the receiver by the cock k (page 207), 

 and its action upon the surface of the quicksilver in the 

 jar will raise it up into the tube, although the piston 

 of the syringe continues motionless. If the tube be 

 about 32 or 33 inches high, the quicksilver will rise in 

 it very near as high as it stands at that time in the ba- 

 rometer. And if the syringe has a small hole, as m, 

 near the top of it, and the piston be drawn up above 

 that hole, the air will rush through the hole into the sy- 

 ringe and tube, and the quicksilver will immediately fall 

 down into the jar. If this part of the apparatus be air- 

 tight, the quicksilver may be pumped up into the tube 

 to the same height that it stands in the barometer ; but 

 it will go no higher, because then the weight of the 

 column in the tube is the same as the weight of a column 

 of air of the same thickness with the quicksilver, and 

 reaching from the earth to the top of the atmosphere. 



7. Having placed the jar A, with some 

 quicksilver in it, on the pump-plate, as in the 

 last experiment, cover it with the receiver B ; 

 then push the open end of the glass tube d e 

 through the collar of leathers in the brass 

 neck C (which it fits so as to be air-tight) 

 almost down to the quicksilver in the jar. 

 Then exhaust the air out of the receiver, and 

 it will also come out of the tube because the 

 tube is close at top. When the guage m m 

 shews that the receiver is well exhausted, 

 push down the tube, so as to immerse its 

 lower end iuto the quicksilver in the jar. 



LECT. 

 VI. 



