292 Evaporation in Facuo* 
with a silver medal for some economical views of this 
subject that I submitted to their consideration. A good 
air pump, is an instrument too expensive and too scarce 
for common use ; and it requires also much attention 
when used. Nor can it be applied to operations in the large 
way. I have therefore endeavoured to supply its place, 
by a method sufficiently plain and simple to be within the 
reach of every body, by exhausting a vessel of air by 
means of steam, and then causing that steam to be absorbed 
by sulphuric acid, by dry pot ash, or by quick lime , and 
preventing the external air from entering the exhausted 
vessel . It is evident that such a vessel ought to remain 
empty of air ; and that the vacuum will be more perfect 
in proportion as the estimate of the air has been more ex- 
act. But I have found by means of several experiments 
made with the utmost c?ire, that by causing a small quan- 
tity of water to boil in a vessel, the air may be expelled 
and a vacuum formed so nearly perfect, that the remain- 
ing air would not occupy a space greater than the 4645th 
part of the capacity of the vessel : an approximation which 
no air pump has yet equalled. Few air pumps even under 
favorable circumstances are able so to rarify the air, as to 
lower the mercury of a manometer half a line below its 
level ; while in air rarified to the degree above mentioned, 
the mercury would stand at 0,073 only above its level. 
(A manometer or manoscope, is an instrument contrived 
to measure the density of the air ; the barometer measures 
only the weight of the air.) 
For the purpose of repeating the well known experi- 
ment of Mr, Leslie, on the congelation of water in vacuo, 
by the means I propose, we may proceed thus. Take a 
bell glass or any glass receiver ; drive out the air by boil- 
ing in it when turned upside down, a small quantity of 
water, or by placing it on a trough full of boiling water, 
or by introducing under it or withinside, any body brought 
