solutions, and from the
effects of bodies put into atmospheres containing the vapours of water, or
camphor, or iodine, &c., as if this attraction were in part elective,
partaking in its characters both of the attraction of aggregation and
chemical affinity: nor is this inconsistent with, but agreeable to, the
idea entertained, that it is the power of particles acting, not upon others
with which they can immediately and intimately combine, but upon such as
are either more distantly situated with respect to them, or which, from
previous condition, physical constitution, or feeble relation, are unable
to enter into decided union with them.

625. Then, of all bodies, the gases are those which might be expected to
show some _mutual_ action whilst _jointly_ under the attractive influence
of the platina or other solid acting substance. Liquids, such as water,
alcohol, &c., are in so dense and comparatively incompressible a state, as
to favour no expectation that their particles should approach much closer
to each other by the attraction of the body to which they adhere, and yet
that attraction must (according to its effects) place their particles as
near to those of the solid wetted body as they are to each other, and in
many cases it is evident that the former attraction is the stronger. But
gases and vapours are bodies competent to suffer very great changes in the
relative distances of their particles by external agencies; and where they
are in immediate contact with the platina, the approximation of the
particles to those of the metal may be very great. In the case of the
hygrometric bodies referred to (621.), it is sufficient to reduce the
vapour to the fluid state, frequently from atmospheres so rare that without
this influence it would be needful to compress them by mechanical force
into a bulk not more than 1/10th or even 1/20th of their original volume
before the vapours would become liquids.

626. Another most important consideration in relation to this action of
bodies, and which, as far as I am aware, has not hitherto been noticed, is
the condition of elasticity under which the gases are placed against the
acting surface. We have but very imperfect notions of the real and intimate
conditions of the particles of a body existing in the solid, the liquid,
and the gaseous state; but when we speak of the gaseous state as being due
to the mutual repulsions of the particles or of their atmospheres, although
we may err in