570  — On the Force of Steam or Vapour 
Mercury and Sulphuric Acid. 
Metcury boils by my thermometer at 660", 
and sulphuric acid of the specific gravity #. 83, 
boils at 590°. It is very difficult to determine 
the precise force of vapour from these liquids 
in any temperature under 212°; because at such 
great distance from the boiling point the vapour 
is so weak as to be in effect almost imperceptible. 
Following the general law, the vapours of these 
fluids ought to be of the force .1, mercury at 
460°, and sulphuric acid at 390°.—Col. Roi - 
makes the expansion of 30 inches mercury by 
180" of heat = .5969 or .5651; and in a baro- 
meter the expansion in the same circumstances 
is .5117; the differences are .0852 and .0534 
which should measure the effective force of mer- 
curial vapour of 212°, nearly, This is in all 
probability too much; as it is next to impos- 
sible to free any liquid entirely from air; and if 
any air enter the vacuum, it unites its force to 
that of the mercurial vapour, 
That the force of vapour from sulphuric acid, 
in low temperatures, is exceedingly small, will 
appear from the ensuing section, 
