Page:Popular Science Monthly Volume 6.djvu/277

Rh their lengths in a common direction, such elongated and flat particles would, when solidified, certainly produce a cleavage.

Plausible as this is, it is not the proper explanation, the cleavage of the slate-rocks being demonstrably not crystalline, but, as shown by Sharpe, Sorby, Haughton, and myself, due to pressure.

The outward forms of these crystals are various and beautiful. A quartz-crystal, for example, is a six-sided prism, capped at each end by six-sided pyramids. Rock-salt, with which your neighbors in Cheshire are so well acquainted, crystallizes in cubes; and it can be cloven into cubes until you cease to be able to cleave further for the very smallness of the masses. Rock-salt is thus proved to have three planes of cleavage at right angles to each other. Iceland spar has also three planes of cleavage, but they are oblique instead of rectangular, the crystal being, therefore, a rhomb instead of a cube. Various crystals, moreover, cleave with different facilities in different directions. A plane of principal cleavage exists in these crystals, and is accompanied by other planes, sometimes of equal, sometimes of unequal value as regards ease of cleavage. Heavy spar, for example, cleaves into prisms, with a rhombus or diamond-shaped figure for a base. It cleaves with greatest ease across the axis of the prism, the other two cleavages having equal values in this respect. Selenite cleaves with extreme facility in one direction, and with unequal facilities in two other directions.

Looking at these beautiful edifices and their internal structure, the pondering mind has submitted to it the question, How have these crystals been built up? What is the origin of this crystalline architecture? Without crossing the boundary of experience, we can make no attempt to answer this question. We have obtained clear conceptions of polar force; we know that polar force may be resident in the molecules or smallest particles of matter—we know that by the play of this force structural arrangement is possible. What, in relation to our present question, is the natural action of a mind furnished with this knowledge? Why, it is compelled by its bias toward unity of principle to transcend experience, and endow the atoms and molecules of which these crystals are built with definite poles, whence issue attractions and repulsions for other poles. In virtue of this attraction and repulsion some poles are drawn together, some retreat from each other; atom is thus added to atom, and molecule to molecule, not boisterously or fortuitously, but silently and symmetrically, and in accordance with laws more rigid than those which guide a human builder when he places his bricks and stones together. From this play of invisible particles we see finally growing up before our eyes these exquisite structures, to which we give the name of crystals.

In the specimens hitherto placed before you the work of the atomic architect has been completed; but you shall see him at work. In the first place, however, I will take one of his most familiar edifices, and