Page:Popular Science Monthly Volume 1.djvu/529



UT of bricks, well burnt, hard, and sharp-angled, lying in heaps by his side, the bricklayer builds, even without mortar, a wall of some height that has considerable stability. With bricks made of bad materials, irregularly burnt, warped, cracked, and many of them broken, he cannot build a dry wall of the same height and stability. The dock-yard laborer, piling cannon-shot, is totally unable to make these spherical masses stand at all as the bricks stand. There are, indeed, certain quite definite shapes into which they may be piled—that of a tetrahedron, or that of a pyramid having a square base, or that of an elongated wedge allied to the pyramid. In any of these forms they may be put together symmetrically and stably; but not in forms with vertical sides or highly-inclined sides. Once more, if, instead of equal spherical shot, the masses to be piled are bowlders, partially but irregularly rounded, and of various sizes, no definite stable form is possible. A comparatively-loose heap, indefinite in its surfaces and angles, is all the laborer can make of them. Putting which several facts together, and asking what is the most general truth they imply, we see it to be this—that the character of the aggregate is determined by the characters of the units.

If we pass from units of these visible, tangible kinds, to the units contemplated by chemists and physicists as making up masses of matter, the same truth meets us. Each so-called element, each combination of elements, each recombination of the compounds, has a form of crystallization. Though its crystals differ in their sizes, and are liable to be modified by truncations of angles and apices, and by partially