1833.] Attraction and Repulsion. 521 
certainly true, that when aqueous vapor was added to the air, it would 
not (according to this doctrine) increase its volume, but this is likewise 
an admission which would destroy the analogy of the experiment ; and 
it even forms an impossibility, with the latter part of the sentence. 
How is it possible that hygrometric vapor (which is an addition of 
matter) should lessen the specific gravity of the air, and yet not in- 
crease its volume ? 
The truth, in short, is, that according to this theory, the hygrome- 
tric vapor could not increase the volume of the air, but then it must 
increase the specific gravity. And since this iscontrary to the physi- 
cal fact, it is manifest, this theory is inadmissible. 
Dr. THomson has adopted the other form of the hypothesis, that li- 
quids pass into vapor solely by their expansive power. He supposes, 
that the vapor and air are not neutral (as was once Mr. Datron’s 
opinion), but elastic towards each other ; and therefore, that water pass- 
es into vapor, although repelled by the air. The following objections 
will, I think, shew, that this theory will not afforda satisfactory explana- 
tion of the fact. It is plain, that the mutual elasticity of the air and 
vapor must be inferior to the elastic force of the vapor of water, other- 
wise the latter could not pass into vapor. But the elastic force of 
aqueous vapor, at most natural temperatures, is not equal to more, than 
35 of the pressure of the air, which pressure must be supported by the 
water, and therefore must press on it with a force far superior to the 
elasticity of vapor, at any natural temperature. And even, if water 
could pass into vapor, this vapor, being lighter than air, would separate 
from it and float above, since it repels the air, unlessthis repulsion be 
exceedingly weak. 
The experiments of Picrer and of Dre Luc, shewing that evaporation 
takes place quite as readily in vacuo, are no proofs, that evaporation in 
the air arises solely from the elastic force of water. They only shew 
(what no one will deny) that the expanding power in water greatly 
aids its evaporation. Water, under the atmosphere, is compressed 
with a force 30 times as great as the strength of its vapor (at most 
natural temperatures) ; there must then exist an attraction between it 
and the air, to enable it to evaporate as much as it does in vacuo, 
when no force is opposing the expansion. 
Lastly then, it would appear that the hygrometric vapor must be at- 
tracted by the air ; and of an attraction between air and water many 
presumptive proofs have been already adduced. 
But, according to the law, that attraction and the force of repulsion 
vary in the same ratio, if the former be superior to the latter force, 
2x 
