1833.] Attraction and Repulsion. 519 
tory, for to each objections may be brought. I shall briefly review them 
separately, and then inquire whether this fact can be explained, on 
the supposition, that the repulsive force of heat varies in the same 
ratio as the attraction. The inquiry will be directed especially to 
the solution of water in the atmosphere, as being the most familiar and 
striking instance. 
First, that aqueous vapor exists in the atmosphere, solely by its own 
expansive force. This hypothesis has had two forms. Mr. Datron 
has supposed, that between different gases the attraction and repulsive 
force of the atoms are so nearly equal, that gases are neutral to- 
wards each other* ; and therefore, that the air has no effect on its hy- 
grometric vapor, which would exist from its own elasticity equally, 
whether the air be present or not. 
Several objections have been brought against this theory, some of 
which Mr. Datron has very ably and ingeniously answered. But 
there are others, which cannot be answered. One of them appears to me 
alone so weighty an objection, as to render it unnecessary to enter in- 
to any besides it,—namely, that if vapor, existing in the atmosphere, 
were perfectly neutral towards it, then certainly the density, and not 
the bulk of the atmosphere, would be increased by the presence of the 
vapor. But Newton has proved that the contrary takes place. 
Aqueous vapor increases the bulk of the air, and even so much as 
to lessen its density. 
If the vapor and air are quite neutral towards each other, how 
could the elastic force of the former act against the atoms of the air 
so as to separate them, which must be the case; for otherwise the 
density would be increased in proportion to the vapor present, instead 
of being diminished ? 
Mr. Dauton has endeavoured to answer this objection by the fol- 
lowing comparison, which I shall attempt to examine, inasmuch as I 
am persuaded it is not applicable, though it has been admitted as such 
by many ; and J may state a refutation of it, which has presented it- 
self to me, and which I do not think has hitherto been proposed. 
** Let” (he says) ‘ a tall cylindrical vessel of glass containing dry 
air be inverted over mercury ; and a portion of the air drawn out by 
a syphon, until an equilibrium of pressure is established within and 
without ; let a small portion of water, ether, &c. be then thrown up 
into the vessel ; the vapor rises and occupies the interstices of the air 
as a void ; but what is the obvious consequence” ? ‘‘ Why,” he says, 
** the surface of the mercury being now pressed both by the dry air 
* New System of Chemical Philosophy, p. 162. 
