EXPERIMENTS ON THE SPECIFIC 159 
vapour, or steam is equally characterized by 
perfect transparency, and it has a degree of 
force, varying with the temperature, by which 
it resists the pressure of the atmosphere. This 
is easily shewn by sending up a drop or two of 
water, ether, &c. into the vacuum of a barom- 
eter. The mercury will descend instantly by a 
quantity greater or less according to the na- 
ture of the liquid and the temperature, without 
any regard to the height of the barometer at 
the time. 
At the temperature of 60° water will depress 
the mercury half an inch; ether thirteen in- 
ches: and these depressions will be the same, 
whether the barometer be high or low; in fact 
an elastic steam instantly arises from the liquid 
and fills the vacuum, exerting its due elasticity 
on the mercurial column. 
If heat be applied to a barometer in such 
circumstances, the liquid which has been pas- 
sed up being wholly converted into vapour, 
there is an increase in the volume of the vapour, 
which follows the same law as that which regu- 
lates the expansion of air in like circumstances. 
But when an excess of liquid is present, every 
degree of temperature adds fresh vapour to that 
