Page:Popular Science Monthly Volume 14.djvu/450

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THE POPULAR SCIENCE MONTHLY.

biology. The psychologist and the theologian are apt to ignore too much the material basis of sociology, the organic laws which through our material nature impress themselves upon the structure and development of society; while the materialist and the political economist are apt to overlook or belittle the importance of the other essential corporations, especially the church, and make sociology nothing more than the highest of the material sciences. But no steady, safe progress in social organization can be made unless we fully recognize the coordinate value of these three, nor in the science of sociology unless we approach the subject from these three sides.

 

THE CRYSTALLIZATION OF GOLD, SILVER, AND OTHER METALS.
By THOMAS J. GREGAN.

THERE are few chemical experiments so well known as the growth of the "lead-tree" and "silver-tree." These carry our minds back to the times of the alchemists, who called the first "Arbor Saturni," and the second "Arbor Dianæ," and they may be looked upon as the type of a large number of phenomena in which the salt of one metal in solution is decomposed by some other metal.

My assistant, Dr. Hand, and myself have lately been experimenting on these replacements, the metallic crystals which are thus produced, and the forces that act through the liquid. Our more special attention has been given to the mutual action of copper and nitrate of silver. If these two substances be brought into contact by the intervention of water, there grows on the red metal what may be called "trees," and, though the analogy between the crystals and the plants is a very superficial one, still the resemblances of external form are sufficiently striking, and a nomenclature drawn from the garden seems the most expressive.

A microscopic view of the growth of these silver crystals round a piece of copper is a truly beautiful sight; a blue glass underneath increases the effect, but they are best seen when they reflect a strong light thrown upon them. They may also be thrown upon a screen as opaque objects, but the beauty and luster of their surfaces are in this way lost. The crystals of silver thus produced differ both in color and form according to the strength of the solution. If it be very weak—say one per cent.—the copper is fringed with black bushes of the metal, which in growing change color to white without any alteration of crystalline form that can be detected by a powerful microscope. A stronger solution gives white crystals from the commencement, which frequently