. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 276 CRYSTALLIZATION. terior of a mass, the question of cleavage suggests itself. In dividing a crystal we create two new surfaces—one on each piece, and each with its own energy. Tlie division nuist tlierefore take place most readily when that surface energy is a minimum. Hence the jmncipal cleavage of a crystal made up of molecules having their motions comprised within spherical spaces will be octahedral. As a fact, we tind that the greate
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 276 CRYSTALLIZATION. terior of a mass, the question of cleavage suggests itself. In dividing a crystal we create two new surfaces—one on each piece, and each with its own energy. Tlie division nuist tlierefore take place most readily when that surface energy is a minimum. Hence the jmncipal cleavage of a crystal made up of molecules having their motions comprised within spherical spaces will be octahedral. As a fact, we tind that the greater part of substances which crystallize in the octahedral, or regular sys- tem, have octahedral cleavage. But not all; there are some, like rock salt and galena, which cleave into cubes, and a very few, like blende? have their easiest cleavage dodecahedral. These I have to explain. I may however first observe that some substances—as, for instance, fluor-spar—which have a very distinct octahedral cleavage are rarely met with in the form of octahedra, bnt usually in cubes. In regard to this, we must remember that the surface energy depends upon the nature of both the substances in contact at the surface, as well as on their electrical condition, their temperature, and other circumstances. The closeness of the molecules in the surface of the solid determines the energy, so far as the solid alone is concerned; but that is not the only —though it may be the most important—factor conducing to the result It is therefore quite possible that, under the circumstances in which the natural crystals of fluor were formed, the surface energy of the cubical faces was less than that of the octahedral, although when we experiment on tliem in the air, it is the other way. This supposition is confirmed by the well-known fact that the form assumed by many salts in crystallizing is affected by the character of the solution. Thus alum, which from a. solution in pure water always assumes the
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