On Water and Air. 
33i 
1880.] 
column of mercury, which is thirteen times heavier than 
water. If mercury is used instead of water, then, instead 
of a column of 32 feet being supported, I ought to have a 
column of only about 30 inches of mercury.*’ I do not know 
any experiment in the whole course of science that must 
have carried more high expectation in its train than this 
experiment of Torricelli. Here is Torricelli’s experiment. 
He filled with mercury a tall glass tube, closed at one end 
(c D, Fig. 36). He placed his finger, as I do, upon the open 
end of the tube, and he turned it upside down. He then 
placed the end closed by the finger underneath the surface 
of some mercury contained in a vessel, and removed his 
finger. You understand the conditions of the experiment. 
When the tube of mercury is thus inverted in a vessel con- 
taining mercury, the mercury is exposed to the pressure of 
the atmosphere. If the pressure of the atmosphere can 
support the whole of that column, the tube will remain full ; 
but if it can only support 30 inches, as Torricelli surmised, 
the mercury will descend in the tube. It will not descend 
quite down, but will stop at a certain point, and then we 
Fig. 36. 
c n 
