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.
EXPERIMENT NO. 1.—Make a strong solution of alum, or of sulphate of copper, or blue vitriol, and place in them rough and irregular pieces of clinker from stoves, or wire baskets, and set them by in a cool place, where they will be free from dust, and in a few days crystals of the several salts will deposit themselves on the baskets, &c. They should then be taken out of the solutions, and dried in an oven not too hot, when they form very pretty ornaments for a room.
EXPERIMENT NO. 2.—Fill a Florence flask up to the neck with a strong solution of sulphate of soda, or Glauber salt, boil it, and tie the mouth over with a piece of moistened bladder while boiling, and set it by in a place where it cannot be disturbed. After twenty-four hours it will probably still remain fluid. Pierce the bladder covering with a penknife, and the percussion of the air will cause the whole mass instantly to crystallize, and the flask will become quite warm from the latent caloric, of which we have spoken before, given out by the salt in passing from the fluid to the solid state. It is better to prepare two or three flasks at the same time, to provide against accidents, for the least shake will often cause the crystallization to take place before the proper time.
THE SPECTRAL LAMP.
Mix some common salt with spirit of wine in a platinum or metallic cup; set the cup upon a wire frame over a spirit-lamp, which should be inclosed on each side, or in a dark-lantern: when the cup becomes heated, and the spirit ignited, it will burn with a strong yellow flame: if, however, it should not be perfectly yellow, throw more salt into the cup. The lamp being thus prepared, all other lights should be extinguished, and the yellow lamp introduced, when an appalling change will be exhibited; all the objects in the room but of one color, and the complexions of the several persons, whether old or young, fair or brunette, will be metamorphosed to a ghastly, death-like yellow; whilst the gayest dresses, as the brightest crimson, the choicest lilac, the most vivid blue or green—all will be changed into one monotonous yellow: each person will be inclined to laugh at his neighbor, himself insensible of being one of the spectral company.
Their astonishment may be heightened by removing the yellow light to one end of the room, and restoring the usual or white light at the other; when one side of each person's dress will resume its original color, while the other will remain yellow; one cheek may bear the bloom of health, and the other, the yellow of jaundice. Or if, when the yellow light only is burning, the white light be introduced within a wire sieve, the company and the objects in the apartment will appear yellow, mottled with white.
Red light may be produced by mixing with the spirit in the cup over the lamp, salt of strontium instead of common salt; and the effect of the white or yellow lights, if introduced through a sieve upon the red light, will be even more striking than the white upon the yellow light.
CURIOUS CHANGE OF COLORS.
Let there be no other light than a taper in the room; then put on a pair of dark green spectacles, and having closed one eye, view the taper with the other. Suddenly remove the spectacles, and the taper will assume a bright red appearance; but if the spectacles be instantly replaced, the eye will be unable to distinguish anything for a second or two. The order of colors will, therefore, be as follows: green, red, green, black.