THE ORIGIN AND STRUCTURE OF METEORITES. 379 



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 py- 

 roxene. If these silicates are in small proportion and thinly scattered 

 through the iron, they are syssiderites ; if it is the iron that is in rela- 

 tively 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. Be- 

 sides stony matters, they contain carbon in combination with hydrogen 

 and oxygen a chemical quality which has led to their being exam- 

 ined 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 pre- 

 sent so complete an identity that their respective fragments can not 

 be distinguished even upon a close mineralogical examination. Noth- 

 ing 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 mal- 

 leability 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 illus- 

 trate, 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 kilo- 

 grammes 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 com- 

 presses the air more rapidly than it can yield, and transmits an equiv- 

 alent motion to its own molecules. Under these circumstances, in the 

 successive detonations caused bv an enforced rotation, iron and the 



