stones as they pass through the air are followed by a tail which, at first bright, soon blends itself with the darkness, like the smoke of a piece of fire-works. It keeps the direction of the path of the meteor for a longer or shorter time, and is undoubtedly composed of particles which have become detached from the body, and remain suspended in the atmosphere till they are scattered by the winds. The mode of the action by which this dust is rubbed or blown off is explained by the experiments I have made with explosive gases in the investigation of the origin of the blister-holes of meteorites. Masses of gas at enormous pressures almost instantaneously pulverize whatever bodies they strike, and this is precisely what happens to the meteoric stones as they pass through the air. Judging from the thickness of the clouds following the bodies and the space they occupy, we conclude that they furnish considerable quantities of metallic and rock-dusts to our atmosphere.
A careful investigation of the dusts which may be supposed to be of cosmic origin is very desirable, as also is a systematic examination of the atmosphere by all the means in our possession, after every explosion of a meteor, for that which they may have left. Something has been done in this direction by Mr. Phipson, M. Nordenskiöld, and M. Gaston Tissandier. Doubtless the shooting-stars, extreme as their tenuity may be, also bring down ponderable substances in minutely divided condition. The spectroscopic examination of these asteroids by Mr. Alexander Herschel has revealed the presence in them of sodium, magnesium, carbon, and other bodies. The fact is confirmed by the formation, in connection with the extraordinary meteoric shower of the 27th of November last, of a cloud of vapors which obscured all the stars except those of the first three magnitudes, and was shortly afterward dissipated.
The question, whether gaseous and invisible substances may not also be introduced to the earth from the realms of space, can not yet be answered from observation.
The most interesting resemblances, and even identities, are occasionally revealed between the meteorites and some of the deeper rocks of our planet.
Volcanoes bring up daily, besides prodigious quantities of vapor of water and gaseous products, melted, intensely hot stony matters, which spread upon their flanks and are known as lavas. During the ancient periods, there came out also, from the depths below the granite, rocks of a nature very different from that of the stratified rocks, and presenting an analogy with the lavas. They occur on the surface in various forms of sheets, cones, and irregular masses. Below it, they constitute, in the thickness of the incasing rocks, a kind of columns, which are connected with the extremely deep reservoirs from which they have been thrown up; they have in fact been thrust here and there in consequence of eruptions, through the superposed masses, far
