t 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