in the cohesive force is called Elasticity, and it exists in no small
degree in glass. The spaces between the particles can, within limits, be
either lessened by compression or increased by distension, and the
particles retain their power of recovering and maintaining the relation
they stood in before they were disturbed. It is the power of cohesion or
aggregation which resists any disturbance among the particles, and which
restores order among them when once disturbance has taken place. And not
only does nature resist directly any undue interference with the
cohering force, but tampering with it even slightly has often a certain
deteriorating effect upon the physical properties of bodies. A bell,
for instance, loses its tone when heated, because by that means its
particles are disturbed; though it recovers its tone-power as it cools,
and as the particles return to their places.

In organic bodies, both during growth and decay, the particles are more
or less in flux; but in feathers, after their formation, the attraction
of aggregation remains constant, and by means of it their particles
continue fixed in their places, not only with the life of the bird, but
long after. Nay, you may even crumple them up, and toss them away as
worthless, and yet if you expose them to the vapour of steam, they will
not only recover their form, but they can be made to look as beautiful
as ever.

_Chemical Affinity_.--The attraction of the particles of bodies of
different kinds to each other is often striking and curious; as, for
instance, those of salt to those of water. The salt attracts the water,
and the water the salt, till at last, if there is a sufficient quantity
of water, all the salt is attracted particle by particle from itself,
and taken up and united to the water. The salt is no longer visible to
the eye, and is said to be dissolved or in solution; but this change of