Fig. 90.
a. The inner cylinder which contains the freezing mixture. b b. The outer one containing spring water. c c. The ice slipping away from the inner cylinder.
In Masters's freezing apparatus certain measured quantities of crystallized sal-ammoniac, nitre, and nitrate of ammonia, are placed in a metallic cylinder, surrounded with a small quantity of spring water contained in an outer vessel. Directly the crystals are liquefied by the addition of water, intense cold is produced, which freezes the water and forms an exact cast of the inner cylinder in ice, and this may afterwards be removed, by pouring away the liquefied salts, and filling the inner cylinder, with water of the same temperature as the air, which rapidly thaws the surrounding ice, and allows it to slip off into any convenient vessel ready to receive it. (Fig. 90.)
For an ingenious method of obtaining large and perfect crystals of almost any size, experimentalists are indebted to Le Blanc. His method consists in first procuring small and perfect crystals—say, octohedra of alum—and then placing them in a broad flat-bottomed pan, he pours over the crystals a quantity of saturated solution of alum, obtained by evaporating a solution of alum until a drop taken out crystallizes on cooling. The positions of the crystals are altered at least once a day with a glass rod, so that all the faces may be alternately exposed to the action of the solution, for the side on which the crystal rests, or is in contact with the vessel, never receives any increment. The crystals will thus gradually grow or increase in size, and when they have done so for some time, the best and most symmetrical, may be removed and placed separately, in vessels containing some of the same saturated solution of alum, and being constantly turned they may be obtained of almost any size desired.
Unless the crystals are removed to fresh solutions, a reaction takes place, in consequence of the exhaustion of the alum from the water, and the crystal is attacked and dissolved. This action is first perceptible on the edges and angles of the crystal; they become blunted and gradually lose their shape altogether. By this method crystals may be made to grow in length or breadth—the former when they are placed upon their sides, the latter if they be made to stand upon their bases.
On Le Blanc's principle, beautiful crystal baskets are made with alum, sulphate of copper, and bichromate of potash. The baskets are usually made of covered copper wire, and when the salts crystallize on them as a nucleus or centre, they are constantly removed to fresh solutions, so that the whole is completely covered, and red, white, and blue sparkling crystal baskets formed. They will retain their brilliancy for any time, by placing them under a glass shade, with a cup containing a little water.
The sketch below affords an excellent illustration of some of Nature's remarkable concretions in the peculiar columnar structure of basalt.
Fig. 91.