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.

There are some metals, such as lead, for instance, which oxydise readily, but this process stops short at the surface in contact with the air, and so forms a coating which prevents the metal from further oxydation; so that here, as in so many things else, strength is connected with weakness.