Many substances when melted and slowly cooled concrete into the most perfect crystals; in these cases heat alone, the antagonist to cohesion, is the solvent power. Thus, if bismuth be melted in a crucible, and when cooling, and just as the pellicle (from _pellis_, a skin or crust) is forming on the surface, if two small holes are instantly made by a rod of iron and the liquid metal poured out from the inside (one of the holes being the entrance for the air, the other the exit for the metal); on carefully breaking the crucible, the bismuth is found to be crystallized in the most lovely cubes. Sulphur, again, may be crystallized in prismatic crystals by pursuing a similar plan; and the great blocks of spermaceti exhibited by wax chandlers in their windows, are crystallized in the interior and prepared on the same principle. There are other modes of conferring the crystalline state upon substances--viz., by elevating them into a state of vapour by the process called sublimation (from _sublimis_, high or exalted), the lifting up and condensation of the vapour in the upper part of a vessel; a process perfectly distinct from that of _distillation_, which means to separate drop by drop. Both of these processes are very ancient, and were invented by the Arabian alchemists long antecedent to the seventh century. Examples of sublimation are shown by heating iodine, and especially [Page 78] benzoic acid; with the latter, a very elegant imitation of snow is produced, by receiving the vapour, on some sprigs of holly or other evergreen, or imitation paper snowdrops and crocuses, placed in a tasteful manner under a glass vessel. The benzoic acid should first be sublimed over the sprigs or artificial flowers in a gas jar, which may be removed when the whole is cold, and a clear glass shade substituted for it. (Fig. 89.) All electro deposits on metals are more or less crystalline; and copper or silver may be deposited in a crystalline form by placing a scraped stick of phosphorus in a solution of sulphate of copper or of nitrate of silver. The phosphorus takes away the oxygen from the metal, or deoxidizes the solution, and the copper or silver reappears in the metallic form. The surface of the phosphorus must not be scraped in the air, but under water, when the operation is perfectly safe.