on Evaporation. 405 
tor producing in it the maximum of evaporation, the inclosed 
barometer, considered here as a manometer, will rise 0,5 
Inch ; as Mr. de Saussure has found it by experiments. 
11. If now we consider that the instrument called barome- 
ter, is in every case a manometer, we shall find, that those 
phenomena observed in close vessels, give us a true idea of 
what happens to steam in the atmosphere. When that in- 
strument is observed in open air, it is with reason called ba- 
rometer ; since the expansive power exercised on its mercu- 
rial column is determined by the weight of the incumbent 
column of air ; therefore it is the true measure of the weight 
of that column. But at the same time, and more immedi- 
ately, it is a real manometer, since the immediate action ex- 
ercised on its column, and to which this is also proportional, 
is, cceteris paribus , exercised by the density of the fluid, or 
fluids, which surround it, whose action is exactly the same as 
if any part of the mass of those fluids were suddenly secluded 
from the rest in a close vessel. Therefore, when steam is 
mixed with air, be the mass shut up in a vessel, or be it a 
certain part of the atmosphere distinct by its place, both fluids 
will act on the column of the manometer or barometer, or on 
every obstacle, and thus against each other, according to their 
respective power ; for this fundamental reason, proceeding 
from the nature of steam as now explained, “ That no me- 
“ chanical cause can produce the decomposition of that fluid, 
<( but by forcing its particles to come. nearer each other than 
<l the actual temperature can permit:” which case cannot happen 
in the atmosphere, except by the accumulation of steam itself 
in some part of it since, elsewhere, it only remains mixed 
mdccxcii, 3 G 
