580 On Evaporation. 
_ The temperature of 144° afford + of the effect 
at boiling ; 138° gave 2 bs GCE 
Having previously to these experiments de- 
termined the force of aqueous vapour at all the 
temperatures under 212°, I was naturally led 
to examine whether the quantity of water eva~ 
porated in a given time bore any proportion to 
the force of vapour of the same temperature, 
and was agreeably surprised to find that they 
exactly corresponded in every part of the ther- 
mometric scale: thus the forces of vapour at 
212°, 180°, 164°, 152°, 144° and 138° are equal to 
30, 15, 10, ya eit and 4 inches of 
mercury respectively, and the grains of water 
evaporated per minute in those temperatures 
were 30, 15, 10, 7%, 6 and 5 also; or numbers 
proportional to these. Indeed it should ‘be so 
from the established Jaw of mechanics, that all 
effects are proportional to the causes producing 
them, . The atmosphere, it ‘should seem, ob 
structs the diffusion of vapour, which would 
otherwise be almost instantaneous, as in vacuo s 
but this obstruction is overcome in. proportion 
to the force of the vapour. The obstruction 
however cannot arise from the weight of the 
atmosphere, as has till now been supposed ; for 
then it would effectually prevent any vapour 
a 
