Brepors- 
ot 
_—y— 
Argument 
om the 
asme quan- 
tty of mois- 
ture being 
ce mtained 
by all the 
gwcs ina 
saturated 
state. 
Doubts con- 
cerning that 
fact. 
. 
218 EVAPORATION. 3 
barometer at 60° passes into vapour, and depresses the 
mercury half an Foch, that vapour. will be completely 
condensed, and the water retamed in the fluid form by 
a pressure a little greater, instead of being able to exist 
under the pressure, which is to that of a column 
of —- 29 inches ae by r ann — wour- 
ed to this objection, by maintaining, what appears 
eho quis rats acne A fact, that pressure does 
not prevent the evaporation of fluids, or at least, that 
the pressure of the atmosphere is exerted in such a way 
as to allow, by its partial action, the escape of some of 
the icles of fluids: for, according to his opinion, 
tee a ore ata een atallle p i 
ticles of any liquid, that the pressure u per- 
i » Prt.ms becomes uniform ; al chet several 
of the particles in the uppermost stratum are in reality 
subject to but little pressure.” It is unnecessary to ob- 
serve that this assertion is entirely hypothetical, and 
even at variance with the known laws of hydrostatics. 
The eae however, receives some degree of 
support the alleged fact, that the same quantity 
of watery vapour is contained in equal volumes of dif- 
ferent gases saturated with moisture. Saussure ascer- 
tained this to be the case with common air, carbonic 
acid, and hydrogen gas, or rather that these gases exhi- 
bited, in the same circumstances, the same ore 
cal condition. Clement and Desormes were also led, by 
their experiments to infer that all gases contain, under 
similar circumstances, equal portions of water. It was 
even stated by Saussure, and the assertion was after- 
wards repeated by De Luc and Dalton, that the quan- 
tity of watery vapour contained in any gas is the same 
as that which would be contained in a vacuum of equal 
extent with the space occupied by the gas. These facts, 
if admitted, seem to favour the opinion that water is 
converted into vapour, and retained in that state, not 
by an affinity subsisting between the water and the gas, 
(or —t - likely <= this ree | — be the same 
with e ,) but merely by the w: vapour 
being raised od action of caloric, aitvahedlints 
diffusing itself mechanically 3 own elasticity. 
On the other hand, it may be said, that the accuracy 
of these facts is, from their extreme delicacy, very ques- 
tionable ; arse sng they have rather been dedu- 
ced by inference, proved by direct experiment. 
A portion of water is allowed to be conv into va- 
pour in vacuo, and the depression which it produces 
— a column of mercury, is assumed as the measure 
of its elasticity: a quantity of dry air is also introduced 
into a receiver of equal capacity with that in which the 
vacuum was formed, and an equal portion of water is 
introduced, the temperature in both cases being the 
same. The scuitoat alaisty of the vapour, indicated 
by the effects of its pressure upon the mercury in an 
inclosed barometer, is the same as that in the vacuum, 
and produces an elevation of half an inch, at the tem- 
peratare of 60°. It is therefore concluded, that an 
equal quantity of water has in all these cases been con- 
verted into vapour. But, in sition to this conelu- 
sion, it has been said, that the elasticity exerted by the 
vapour under these different circumstances, is not an 
exact test of its quantity, unless it be admitted-that the 
vapour is not all attracted by the gases ; and this is the 
eo point to be established. If the vapour be eombi- 
with the gas, its elasticity will be counteracted in 
a degree proportional to the attraction exerted; and it 
is easy to conceive, that, at the point of saturation, the 
elasticity may be so modified by the power of affinity, 
as to indicate the same degree of intensity, though the 
quantities of vapour which are present be very differ- a 9 
ent. 
But it has also been urged, casa wr er i 2 
, that the quantity of moisture “by equal 
oreo i t forsscmyppe tone a pwd aps oe 
cases, the same when they are to substances 
as might naturally be expected from the e of 
daaieg the experiments. But though the ar; st 
afforded by these experiments, in support of opi- 
nion that the vapour exerts no action with the gases, 
must be admitted to be of some weight, it be 
regarded as perfectly conclusive ; since~it still remains 
to be determined, whether . the quantities of water, 
which cannot be taken from the different gases by de- 
siccation, are exactly equal ; and there appears to be 
no means of ascertaining this by direct experiment. 
It cannot be doubted, that the gases do differ from 
one another, with respect to their affinity for water-; for the 
pee BL TIRE RE op ne > OH oY s, and 
tains them in combination wi Secisiontte 
so e of di ing water in a 
ing u this affinity, though modified, no doubt, 
the elasticity of the vapour which is formed. Mr 
ton maintains, that the combination is entirely mechan- 
ical ; and that the absorbed gas received into the vacant 
ui 
ound, that the temperature being the same, the quan- 
tty of any gas absorbed by water, was exactly pro- 
portional to 
ces of temperature, water takes up, in all cases, the 
same volume of condensed gas, as of gas under ordi« 
nary pressure.” But admitting the law, as Dr Murs 
ray has stated with his usual ingenuity, the conclusion 
absorption of gas by water, two powers or may 
be conceived to ate, independent of pressure,—the 
affinity between gas and water to combine 
> . this, 
and’ placing limits to the peri ago Sere in the 
frees operating the chemi fit, andthe ohesion 
forces. . i ity, ion 
of the solid. In the absorption ‘of a gas, whatever fa- 
vours the exertion of the. elasticity will. lessen. the 
— absorbed ; whatever represses it, will promote 
the absorption. These effects are produced by varia- 
i 
; 
