Aerolites are composed of the same elementary substances as occur in terrestrial minerals, not a single new element having been found in their analysis. Of the sixty or more elements now recognized by chemists, about twenty have been found in aerolites. While aerolites contain no new elements, their appearance is quite peculiar; and the compounds found in them are so peculiar as to enable us by chemical analysis to distinguish an aerolite from any terrestrial substance. Iron ores are very abundant in nature, but iron in the metallic state is exceedingly rare. Now, aerolites invariably contain metallic iron, sometimes from ninety to ninety-six per cent. This iron is malleable, and may be readily worked into cutting instruments. It always contains eight or ten per cent of nickel, together with small quantities of cobalt, copper, tin, and chromium. This composition _has never been found in any terrestrial mineral_. Aerolites also contain, usually in small amount, a compound of iron, nickel, and phosphorus, which has never been found elsewhere. Meteorites often present the appearance of having been fused on the surface to a slight depth, and meteoric iron is found to have a peculiar crystalline structure. The external appearance of a piece of meteoric iron found near Lockport, N.Y., is shown in Fig. 350. Fig. 351 shows the peculiar internal structure of meteoric iron. [Illustration: Fig. 350.] [Illustration: Fig. 351.] 308. _Meteoroids._--Astronomers now universally hold that shooting-stars, meteors, and aerolites are all minute bodies, revolving, like the comets, about the sun. They are moving in every possible direction through the celestial spaces. They may not average more than one in a million of cubic miles, and yet their total number exceeds all calculation. Of the nature of the minuter bodies of this class nothing is certainly known. The earth is continually encountering them in its journey around the sun. They are burned by passing through the upper regions of our atmosphere, and the shooting-star is simply the light of that burning. These bodies, which are invisible till they plunge into the earth's atmosphere, are called _meteoroids_.