Page:Popular Science Monthly Volume 74.djvu/583

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HEN I was a boy in New England it was the fashion to decorate the windows of the drug-stores with masses of huge deep blue crystals of copper sulphate—blue-vitrol or blue-stone. These ornaments have vanished now, even from the country drug-stores. Their places are taken by electrical toys, patent medicines, even animals. But these lumps of translucent crystals always interested me. Their composition is simple; sulphate of copper and water are their sole constituents. The sulphate of copper is itself a dull gray powder, not a crystalline substance at all; but if water is with it, it becomes blue, colors its solutions blue, and crystallizes in very regular form. The color and the form of these crystals depend, therefore, upon both water and copper sulphate.

The boy who plays with blue-stone, dissolving it in water and then recovering it again by crystallization, thus doing for fun what the freshman in a chemical laboratory does because he is directed, learns that the crystals will be large or small, few or many, according to the speed with which they form; large and few if they form slowly, small and numerous if they form rapidly. This is equally true of sugar, common salt and other crystalline substances. We see, then, that circumstances as well as substance have to be taken into account. Although we can not have crystals of this particular kind unless we have the sulphate of copper, neither can we have them, even with an abundance of the salt, unless we have water also. The water must not be in excess, lest the copper salt remain in solution; nor deficient, lest it remain amorphous. It must be exactly proportioned in quantity if the salt is to arrange itself into bodies of definite form. The size and the number of these bodies depend upon temperature, dryness of air, any circumstance, in fact, which influences the rate at which water evaporates from the solution.

Although the number, size and even the formation of blue-stone crystals depend upon circumstances, and will vary according to circumstances, circumstances can not make blue-stone crystals exactly like the crystals of other things. The crystals of common salt have characters which identify them to the eye and mind of the crystallographer. These characters, we say, are inherent in the substance itself. This may be true actually as it certainly is true practically; but a scientific man might be found who would hazard the opinion that common salt, which crystallizes in square plates with hollowed surfaces under the conditions which we know, might crystallize in