The greater number of salts have a tendency to assume regular forms, or become crystallized, when passing from the fluid to the solid state; and the size and regularity of the crystals depends in a great measure on the slow or rapid escape of the fluid in which they were dissolved.
Sugar is a capital example of this property; the ordinary loaf-sugar being rapidly boiled down, as it is called; while to make rock-candy, which is nothing but sugar in a crystallized form, the solution is allowed to evaporate slowly, and as it cools it forms into those beautiful crystals termed rock-candy. The threads found in the center of some of the crystals are merely placed for the purpose of hastening the formation of the crystals.
Restoration of Color by Water.
Water being a colorous fluid ought, one would imagine when mixed with other substances of no decided color, to produce a colorless compound. Nevertheless, it is to water only that blue vitriol or sulphate of copper owes its vivid blueness, as will be plainly evinced by the following simple experiment. Heat a few crystals of the vitriol in a fire-shovel, pulverize them, and the powder will be of a dull and dirty white appearance. Pour a little water upon this when a slight hissing noise will be heard, and at the same moment the blue color will instantly reappear.
Under the microscope the beauty of this experiment will be increased, for the instant that a drop of water is placed in contact with the vitriol, the powder may be seen to shoot into blue prisms. If a crystal of prussiate of potash be similarly heated its yellow color will vanish, but reappear on being dropped into water.
Two Liquids Make a Solid.
Dissolve chloride of lime in water until it will dissolve no more; measure out an equal quantity of oil of vitriol; both will be transparent fluids; but if equal quantities of each be slowly mixed and stirred together, they will become a solid mass, with the evolution of smoke or fumes.