acts only when the
particles seem almost in contact, and it ceases altogether when once, by
mechanical or other means, the bond is broken, in consequence of the
particles being forced too near, or sundered too far from, one another.

One distinguishing difference between the attraction of gravitation and
that of cohesion is, that whereas the former is uniform, the latter is
variable; that is, under gravitation the attraction of any one particle
to any other is the same, but under cohesion, some sets of particles
are more forcibly drawn together than others. For instance, a particle
of iron and a particle of cork gravitate equally, but particles of iron
and particles of cork among themselves do not cohere equally. And it is
just because those of the former cohere more than those of the latter,
that a piece of iron feels harder and weighs heavier than a piece of
cork.

Further, the attraction of gravitation is unaffected by change in the
condition of bodies, while that of cohesion is. It makes nothing to
gravitation whether a piece of metal is as cold as ice, or heated with a
sevenfold heat. Not so to the power of cohesion; withdraw heat, and the
particles under cohesion cling closer; add it, and both the spaces grow
wider and the attraction feebler. Thus, for example, you may suspend a
weight by a piece of copper-wire, and the wire not break. But apply heat
to the wire, and its cohesion will be lessened; the force of gravitation
will overpower it, rupture the wire, and cause the weight to fall.

_Cohesion_.--That the action of the attraction of cohesion depends on
the contiguity of the particles in the cohering body, may be shown by
an illustration. Take a ball of lead, divide it into two hemispheres,
smooth the surfaces of section, then press them together, and you will
find it requires some force to separate them; thus proving the
dependence of cohesion on contiguity, although the effect in this case
may be due in some degree to the pressure of the atmosphere as well as
the power of cohesion.

Heat is the principal agent in inducing cohesion, as well as in relaxing
its energy; for by means of it you can weld the hardest as well as the
softest substances into one, and two pieces of iron together, no less
than two pieces of wax. It is possible, indeed, by heat to unite two
sufficient waxed corks to one another, so as to be able by means of the
one to draw the other out of a bottle: such, in this case, is the force
of