Page:Popular Science Monthly Volume 37.djvu/387

Rh out during a slow process of crystallization, evidently from a melted condition. This structure is best seen on a polished surface which has been subject to tempering, or else etched with acid. The acid, acting most readily upon the purest parts of the iron, develops certain figures called Widmanstättian figures if the plates are broad and well marked, and called Neumann lines where they are reduced to fine markings. Till recently, these two varieties of etched figures were supposed to indicate a difference of crystalline structure; but, by a study of the Harvard collection (American Journal of Science, third series, vol. xxxii, p. 284), it has been shown to depend on the time of crystallization—that is to say, on merely the size of the crystals, and not a difference of form. In some cases these etched figures serve conclusively to distinguish irons of different falls, but frequently they vary on the same specimen, or depend on the direction in which the surface is cut; but there are large groups of irons closely resembling each other in their etched characters. The distinction of such irons has become of great importance, since the enormous prices paid for meteorites offers a strong inducement to multiply supposed falls. Iron meteorites are often cut up and distributed by the finders before they have been fully identified, and the confusion is further increased by the natural distribution due to the explosions in the upper atmosphere. Thus a meteorite which fell in Cocke County, Tennessee, some time previous to 1840, has been turning up at various places ever since, and the numerous fragments have been described from time to time under various names as different falls.

In an attempt to prove that an iron which was found in Maverick County, Kentucky, was identical with two Mexican ones, in the Harvard collection (Proceedings of the American Academy of Arts and Sciences, vol. xxiv, p. 30), the writer found that on breaking slabs of the respective irons the two Mexican specimens, which had been generally accepted as identical, showed a marked difference of structure. One of them, known as the Butcher iron, when broken in various directions by blows of a hammer, always exhibited most brilliant and complex crystal faces, some of them half an inch in diameter, certain of the faces being most beautifully marked by a system of fine parallel lines arranged at certain fixed angles. The second iron, on the other hand, from Santa Rosa, would only break in two definite directions, exhibiting a single face with little flaky surfaces, but none of the fine lines. This last iron, if sawed to a thin edge, and then forced to break in a different direction from the two just mentioned, showed only a series of little cube faces, very different from the Butcher iron.

On a similar examination of other irons resembling the two Mexican ones in the figures brought out by etching, irons from