Different 
theories of of explaining the conversion of water into 
grapecstion: gucbteral tors , and its 
EVA 
that the generalisation arose from the repeated applica- 
tion of the same sound, to express individual objects or 
individual feelings, concurring in the same common 
wethtieet aupaiapsechestapeaen diene 
ight be f ing wi progress of na- 
ture, and an excellent track would be opened, for tra- 
cing the procedure of the human intellect in the ar- 
rangement of ideas and formation of speech. A com- 
parison of these primitive roots, too, would best enable 
us to determine what degree of affinity existed among 
the languages to which they belong, and, perhaps, 
throw some light on the much-agitated question, 
whether all the of the world were really de- 
rived from one. Probable as the opinion of their com- 
origin must be allowed to be, and ened 
ing proofs of actual coincidence, still, to 
'y, such evidence seems to be want- 
acetone wascipionion: men- 
Alexandrian school, but these are only incidentally, 
eeeeeoeniely eat ste In modern 
Rave distinguished them. 
selves in this line. Horne Tooke has thrown great 
light on j wa Jamieson has fur- 
Seapasgen:.‘Thaiecpected wurksofs-t Mann pen 
ern, F ex work o urray on 
the languages of Europe, there is reason to , will 
a most valuable acquisition to the lovers of etymo- 
logical research 0) 
. EVAPORATION, is the process by which the aque- 
ous particles of bodies are converted into vapour. This 
is of two kinds, Artificial, and Natural or 
taneous. We have already described the general 
appearances connected with the former, (See Cuemis- 
“ry, p. 37.), and we shall therefore confine our atten- 
tion at present to the consideration — latter. 
Spontaneous evaporation is prombted by a variety of 
causes, of which the principal appear to be elevation of 
temperature, and the successive application of fresh 
portions of air. Neither of these circumstances, how- 
ever, can be deemed essentially necessary to the pro- 
cess; since ice gtadually wastes away at a very redu- 
eed and eyen more rapidly under an ex- 
pr erm it is exposed to a current 
atmospheric air. 
- Various theories have been proposed, with the view 
vapour, at 
elevation in 
the ; but notwiths 
with which the process is presented’ to our observation, 
and the important which it is destined to 
serve in the economy of nature, there are few subjects 
of ical enquiry involved in greater obscurity. 
According to Des Cartes, the action of the sun 
the water converts small particles of that fluid into hol- 
VOL, IX. PART I.” : wt 
217 
in the a 
‘EVA 
low spheres, which, being filled with a subtile matter, 
are rendered specifically lighter than air, and thus 
ascend in the atmosphere. iers, assuming the 
hypothesis that heat acts more powerfully on water 
than on common air, asserts that at a temperature which 
-by. its cold condenses air, may be sufficient to cause an 
evaporation from water, or even from ice. He farther 
assumes, that the particles of water, after they are con- 
verted into vapour, acquire a repellency to one another, 
and deriving elasticity from the contiguous air, recede 
farther and farther till the specific gravity of the fluid 
which they form becomes lighter than air, after which 
they ascend. The particles of the vapour are supposed 
to retain their repellent force till by the diminution of 
the density of the surrounding air, their relative weight 
ra and they again descend in the form of rain, 
, Se. ‘ 
A theory somewhat similar to this has been support- 
ed, with much ingenuity, by Dalton and De Luc. 
These philosophers maintain, that since. water passes 
readily into vapour in vacuo, where the agency of the 
air is com 'y excluded, its spontaneous evaporation 
tmosphere may be referred entirely to the ope- 
ration of calorie. Accordingly, Mr Dalton, agreeably to 
-his opinion respecting the constitution of the atmo- 
sphere, asserts, that the aqueous vapour, thus formed 
by heat alone, exists in air, not in a state of combina- 
tion with it, but merely of mixture or diffusion ;—that 
it exerts no action whatever with the surrounding ga¢- 
es, but supports itself entirely by its own elasticity ;— 
and that the quantity of it ds entirely upon th 
temperature of the air, and the pressure exérted by the 
vapour already formed. He even advances a step be- 
yond this, and affirms that the quantity of vapour which 
could exist in the atmosphere would be the same, though 
the pressure of the atmosphere did not exist, as the va- 
pour itself would soon accumulate, and form an atmo- 
which would uce on the surface of liquids 
the mechanical effects of the air itself. 
The princi t which Mr Dalton has brought 
forward in support of his theory, is drawn from the 
well-known fact, that water passes readily into vapour 
under an exhausted receiver, and where, since there is 
no air the evaporation must be ascribed entire- 
ly to the influence of caloric. Dr Murray has shewn, 
however, that it by no means follows, because water 
into vapour in vacuo, at-a natural temperature, 
it will pass into vapour to the same extent, at the same 
temperature, under the ordi pressure of the atmo- 
wt The two cases are indeed totally different, 
en water is placed in vacuo, evaporation cannot, it 
must be admitted, be owing to the presence of air; but 
it ought to be recollected, when the air is removed, 
its mechanical pressure is removed along with it: and 
it would be altogether illogical to conclude, that because 
water passes into in the absence of that pres- 
sure, it would do so in an equal degree when 
to it. “« The proper manner of making the experiment,” 
as Dr Murray very properly remarks, “is to exclude 
e air, 
the chemical while the pressure of it 
is preserved; in other words, to place: water in vacuo 
under the re of a column of mereury 29 inches in 
height, which is equal to the presse ‘of the atmo- 
sphere. If in this case any vapour were forined, the 
conclusion would be just, that eous evaporation 
is i ent of any chemical agency of atmosphe- 
Evapora- 
tion. 
—\ye 
Theory of 
Dalton and 
De Luc, 
ric air, But the fact will be found very different; , 
for although water introduced into the tube of the 
i to ee 
