Page:Popular Science Monthly Volume 14.djvu/454

438 also, there is a luxuriant growth of large, flat leaflets, or symmetrical structures resembling fern-leaves, but with the fronds arranged at right angles, or combinations of these with octohedra, as shown in Fig. 8, These fern-leaves often begin of a dull-gray color, but, as they advance, suddenly change to a brilliant white. The particular form of these crystalline growths depends, therefore, primarily on the specific character of the metal, but this is greatly modified by the strength of the solution.

The forms assumed by native metals resemble those produced by the process of substitution. In some cases, indeed, it seems almost certain that the deposition of these minerals was effected in the same way—as, for instance, the silver on the native copper of the Lake Superior district.

Gold is frequently found in cubes more or less rolled, but the leaf-gold from Transylvania bears a striking likeness to the crystals that form in our laboratory experiments.

Silver is often found native as twisted hairs or wires of metal—a form that never occurs in the decomposition of its nitrate by copper, but which can be artificially produced in another way. There has been noticed a singular tendency in old silver ornaments and coins to become crystalline and friable. I have an ancient fibula from the island of Cyprus, supposed to be at least fifteen hundred years old, which, through the greater portion of its substance, presents a fracture something like that of cast iron, and its specific gravity has been reduced in round numbers from ten to nine.

It contains a little copper. This property of certain metals, or their alloys, to change in condition and in volume, is worthy the attention of those whose duty it is to make our standards. Experiments should be instituted for the purpose of learning what metals or combinations of metals are least subject to this secular change.

These metallic crystals are Nature's first attempt at building. The material is the simplest possible—in fact, what chemists look upon as elementary. But how is the building carried on? What are the tools employed? Where are the bearers of burdens that bring and prepare the pieces and lay them together according to the plan of the Great Architect? We must imagine what is taking place in the transparent solution. The silver, of course, existed at first in combination with the nitric element, and for every particle of silver deposited on the growing tree an equivalent particle of copper is dissolved from the surface of the plate. The nitric element never ceases to be in combination with a metal, but is transferred from the one metal to the other. On the "polarization theory" the positive and negative elements of