being affected by the acid, stands out in relief from the other, which is attacked by it. The meteorites of this group are called holosiderites, or all iron, in distinction from the others, which contain also stony matters. They are vastly more rare than those of the other groups. The stony substances of the other groups consist chiefly of silica in combination with magnesia and peroxide of iron, as peridote or pyroxene. If these silicates are in small proportion and thinly scattered through the iron, they are syssiderites; if it is the iron that is in relatively small proportion and appearing only in isolated grains, they are sporadosiderites. In other meteorites, comparatively few in number, no metallic iron can be perceived, and they are called asiderites. The most interesting specimens among them are remarked by their dull-black color, and a general appearance like that of peat or lignite. Besides stony matters, they contain carbon in combination with hydrogen and oxygen—a chemical quality which has led to their being examined for remains of organic beings. But no trace of anything of the kind has been discovered. They also have escaped all alteration by heat beyond the superficial glazing, and thus strengthen the evidence that their origin is exterior to our globe.
Among all the diversities presented by the specimens of more than four hundred meteoric showers, is the remarkable fact that meteorites which have fallen at the most distant epochs, and in countries most remote from one another, not only conform to the same type, but present so complete an identity that their respective fragments can not be distinguished even upon a close mineralogical examination. Nothing in the exterior form of meteorites is more striking than a general aspect indicating that they are parts of a broken body. When we compare hundreds or thousands of stones of the same fall, we find that they all present polyhedral forms like those of stones broken for a macadamized road, except that the angles are more or less rounded. Even meteoric iron exhibits this angular shape, showing that its malleability and extreme tenacity have not preserved it from a violent rupture. It seemed impossible that such an effect could be produced solely by the action of the air, especially in the upper regions where it is in an extremely rarefied condition. But light has been thrown upon the problem since the introduction of the new explosives, which illustrate, in their industrial applications, the prodigious force that gases are capable of exerting, even in small quantity, when they are suddenly animated by a considerable tension. The explosion of a kilogramme of dynamite will break up bars of steel which a pressure of a million kilogrammes would hardly crack. Similar conditions concur in the upper strata of the atmosphere, slight as their density may be, when a meteor moving with planetary velocity strikes upon them. The body compresses the air more rapidly than it can yield, and transmits an equivalent motion to its own molecules. Under these circumstances, in the successive detonations caused by an enforced rotation, iron and the
