ioo Faraday's Researches

359. Extraneous bodies, which, acting as nuclei in crystallising
or depositing solutions., cause deposition of substances on them,
when it does not occur elsewhere in the liquid, seem to produce
their effects by a power of the same kind, i.e. a power of attrac-
tion extending to neighbouring particles, and causing them to
become attached to the nuclei, although it is not strong enough
to make them combine chemically with their substance.

360. It would appear from many cases of nuclei in solutions,
and from the effects of bodies put into atmospheres containing
the vapours of water, or camphor, or iodine, etc., 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.

361. 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, etc., are in so dense and com-
paratively 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 (357), 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 one-tenth or even one-twentieth
of their original volume before the vapours would become liquids.

362. 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 * Ibid. torn, xxiii. p. 440; torn. xxiv. p. 380., not the slightest tendency to the