Page:Popular Science Monthly Volume 29.djvu/392

378 by the moon and the earth. That velocity should be at least 2,270 metres a second, or about five times that of a cannon-ball; if it were less, the mass would fall back to the moon. Another more probable supposition is that they come from a group of minute asteroids which revolve in the space between Mars and Jupiter, whose orbits cross those of the large planets, ours included, and are occasionally met by the earth in its course. There is nothing else, since the beautiful researches of Schiaparelli have connected the periodical swarms of shooting-stars with comets, to assure us that they do not come from still more distant parts of the sky, or even from without the solar system.

Shooting-stars come to us by millions at regular periods; and the number of those which are directed toward our globe in a single year is estimated at many milliards. They somewhat resemble meteorites in the abruptness of their appearance in our atmosphere and the excessive rapidity of their motion, but they differ from them in an important characteristic. None of them ever reach the ground. They appear to share in the properties of comets, from which they may have been dismembered and told off by perturbing actions; while meteorites seem to be related to the planets. The difference between them is like the difference between gases or vapors and solid bodies.

The meteors coming to our earth without, excepting as to their superficial vitrification, undergoing any change, we are able, by subjecting them to analysis, to derive from them some precise facts respecting the constitution of the bodies in space. The first fact, which comes out from hundreds of analyses, is, that they have not brought a single substance which is foreign to our globe. About twenty-two elements, all known to the chemistry of the earth, have been recognized as present in them. Among these, iron, silicon, magnesium, nickel, sulphur, phosphorus, and carbon, are the most important. While they are all clad externally in a common livery, meteorites, when examined in their fractured parts, along with traits of similarity, present considerable differences. They have been classified, according to their types, into four groups, according to the proportion of iron they contained. Those of the first group are composed almost wholly of iron, which is known as meteoric iron. It is always alloyed with nickel and a few other metals, and contains carbon free or in combination, as in steel, with frequently sulphuret and phosphuret of iron in scattered globules and grains. It is always recognizable by a single peculiarity in its structure. If we moisten a polished surface of it with an acid, we shall immediately observe the appearance of numerous straight lines, as fine and as true in their parallelism as if made with an engraver's tool, and crossing one another in a net-work of regular geometrical figures. These designs, called the figures of Widmanstaetten, after the first observer of them, result from the fact that the metal is not of homogeneous constitution. It is composed of two alloys of iron and nickel, in a crystalline condition, one of which, not