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Page 26
There are some liquids the adhesiveness of whose particles is so perfect
as to bar out the access of air when we strew them on the surface of
other liquids; and on the Continent it is not uncommon to protect wines
against the action of the atmosphere by, instead of corking the bottle,
simply pouring in a few drops of oil, which, being lighter than the
wine, floats on the surface. It is parallel to the instance of the
barrel with the gauze-wire top mentioned above, that if we loosely plug
a bottle full of liquid with a piece of cotton-wool, and invert it, the
particles in contact with the wool will cohere so closely that the fluid
will not be able to escape. The adhesiveness of the particles of water
to a solid surface can be exemplified by allowing one of the scales of a
balance to float in water and leaving the other free; the one in
contact with the water will refuse to yield after we have placed even a
tolerable weight in the other which is suspended in the air.
The power of cohesion is more rigorous in some bodies than others. In
some cases the body will rupture if it is interfered with ever so
little; in others, the particles admit of a certain displacement, and if
the limits are not transgressed, they return to their original position
when the compressing or distending cause is removed. This rallying power
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
form is due to its affinity for the water, and the resulting attraction
of the one to the other. The same phenomena are observed, and they are
due to the same cause, in other solutions; as when we infuse our tea or
sweeten it with sugar. The attraction of water, or one of its elements
rather, for other substances, sometimes shows itself in vehement forms.
When a piece of potassium, for example, is thrown into a vessel of
water, its attraction for the water is such, and of the water for it,
that it instantly takes fire, and the two blaze away, particle violently
seizing on particle until the elements of the water unite part for part
with the metal. It is the mutually attractive force that causes the heat
and flame which accompany the combination; and this force is most
violently active in the union of dissimilar substances. Unions of a
quieter kind, though not less thorough, occur even between solids when
placed in contact. For instance, sulphate of soda and sulphate of
ammonia, when placed side by side, will diliquesce, and in liquid form
unite into a new combination. Sulphuric acid, when we mix it with water,
generates great heat; and this is due to its attraction for the water.
Sometimes two fluids unite together, and, in doing so, pass from the
liquid into the solid form; as, _e.g._, sulphuric acid and chloride of
calcium. Attraction of this nature is called chemical: it takes effect
between dissimilar particles, and results in combinations with new
properties. It operates not only between solid and solid, solid and
liquid, and liquid and liquid, but between these and gases, and gases
with one another; and these as well as those combine into new
substances, and evince in the act not a little violent commotion. Thus,
phosphorus catches fire in the atmosphere at a temperature of 140
degrees, and it goes on rapidly combining with the oxygen, burning with
a dazzling white light, and producing phosphoric acid. Indeed, most
metals have an affinity for the oxygen in the air, and oxydise in it
with more or less facility; and a metal, as such, has more value than
another according as it has less affinity for that element, and is less
liable to oxydise or rust in it. This is one reason, among others, why
gold is the most precious metal, and the conventional representative of
highest worth in things.
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