One of the most beautiful illustrations of cohesive attraction is seen in crystallization. In every instance in which substances pass into the form of a solid, they tend to assume regular shapes called crystals. Each material has its own characteristic form, so that a crystal is a type of a species in the mineral world, even as a plant or an animal is in the organic kingdom. A crystal is a substance bounded by plain surfaces and symmetrically arranged about imaginary lines called axes. The final form depends upon certain smaller forms in its interior structure. They possess lines of division, often in three directions, called “cleavage.”
While there are millions of crystals, they have all been classified under six systems, as follows: 1. Monometric, where the three axes are equal. 2. Dimetric, having one axis unequal to the other two, which are equal to each other. 3. Trimetric, having no two axes equal. 4. Monoclinic, having one axis inclined. 5. Triclinic, in which all the three intersections are oblique and the axes unequal. 6. Hexagonal, which has the form of a regular hexagonal prism.
[Illustration: _Ex._—Showing change of volume. The upper part of the figure represents a substance expanded. There are no more molecules here than below, but they are pushed further apart. This is supposed to be the way in which all bodies are enlarged by heat.]
While contemplating the thousand beautiful forms in which molecules are arranged into crystals, whereby many economic purposes are served, as well as taste manifested, one can not resist the conviction that such displays of wisdom, benevolence and love of beauty can alone emanate from the eternal Mind.
Another wide-spread effect of cohesion is seen in
Everywhere in fossiliferous rock may be found organic remains in which the material of which they were originally composed has been replaced by some mineral substance. Some have supposed that these plants and animals have actually been converted into stone by a change of their elements. This is of course absurd. Carbon can never be anything but carbon, nor indeed, can any element ever become anything other than itself. This dream of the alchemist was long since dissipated. No, strange as it may seem, the molecules of these fossilized organisms must actually pass out, and silica, lime, clay, or some such matter pass in and take their places. Beautiful specimens of petrified wood, found especially on the Pacific coast, are often hard as glass. One very handsome variety, called “opalized” wood, clearly indicates that petrifaction was either accompanied or followed by crystallization.