Page:Popular Science Monthly Volume 8.djvu/481

Rh The members of any one of these groups resemble each other chemically in the closest manner, forming compounds of strong similarity, and often are very much alike in their physical properties also. The thought at once arises, Can these elements be totally distinct from each other—have they nothing in common—are these resemblances only due to chance? Such a supposition could scarcely be admitted, since Science excludes chance from her list of natural agencies. These relationships must mean something—but what?

If we look beyond the points of similarity to the points of difference between related elements, we shall find that these too are subject to regularity. The members of a group vary from each other, not in a meaningless, helter-skelter way, but systematically, so that they may be arranged in regular series. Take, for example, the group formed by the strikingly similar metals, calcium, strontium, and barium. If, now, we compare these with reference to any physical property, we shall find that strontium will always be between the other two. It is heavier than calcium and lighter than barium; and the same thing holds true of strontium compounds when compared with the corresponding compounds of its two associates. The integrity of the series is perfect; for in no case can the middle member be placed either at the beginning or the end. The nitrogen group is even more remarkable. Arranging its recognized members in the order of their atomic weight, they are as follows: nitrogen, phosphorus, arsenic, antimony, and bismuth. The first of these elements is gaseous at all known temperatures; phosphorus is a solid, but easily convertible into a gas by heat; arsenic is a denser body still, and less readily vaporized; antimony follows in regular order; and finally, bismuth, the heaviest of the series, can be distilled only with considerable difficulty. Here, then, is a gradation both in specific, gravity and in boiling-point, the lowest member of the group, in each of these particulars, being that with the lowest atomic weight; and the reverse. If we ascend from these elements to their compounds, we shall also notice some curious chemical regularities. Each member of the group unites with oxygen to form a pentoxide, from which an acid may be derived. Compare, now, these five acids: nitric is very strong, and violently corrosive; phosphoric is a little weaker, and acts much less vigorously; arsenic is feebler still; antimonic is extremely weak; and the corresponding bismuth compound is just barely recognizable as being an acid at all. Can these regular gradations be purely accidental and meaningless?

Examples like these might be adduced almost indefinitely. Series after series could be brought forward, all illustrating the same principle. Exceptions occur now and then, but they are so few that for present purposes they may be disregarded. Of course they mean something, but they are neither sufficiently abundant nor important enough to affect our arguments. The regularities are so numerous and so