Page:Popular Science Monthly Volume 29.djvu/394

380 most tenacious bodies will fly into pieces, as if they were struck with a pile-driver.

There is another no less characteristic feature of the surface of meteorites which testifies to the violence of the mechanical action produced upon them by the atmospheric rebound, exhibited by rounded cavities resembling finger-marks. They appear in the stony meteors, but are particularly characteristic of the iron masses. These marks were at one time attributed to transient explosions taking place during the course of the meteor through the air; but experiment has shown that the same appearance is produced in bodies which are acted upon by an explosion of dynamite; in the grains of coarse powder that drop, half consumed, from the mouth of a cannon when it is fired, and upon the touch-hole of the cannon. They are all due to the same cause to the erosive action of gas revolving rapidly and moving spirally and under high pressure against the projectiles, boring into them as if it were a gimlet. The mechanical action is accompanied and aided by a chemical action which is dependent upon the combustible nature of iron at high temperatures. Although these blister-holes are worked only on the face which is exposed to the direct pressure of the gas, meteorites present them on various sides, and sometimes over their whole surface. This arises from the rotatory character of the motion of the body which makes it present every side in succession to the front.

With these mechanical phenomena of meteorites is connected the coming to the earth of dusts of celestial origin. In examining these, we must first be careful to separate the earthly dusts, with which the air is more or less loaded, of every kind, natural and artificial, from volcanoes and from waste tracts of the earth's surface, mineral, vegetable, and animal. These are recognizable by careful examination; and, after they are all detected, there remain still other dusts, which incontestably come to us from regions foreign to our globe. The carbonaceous meteorites of Orgueil furnish us a very interesting prime document respecting them. These bodies are so friable that they are reduced to powder under the slightest pressure of the fingers, and they would probably have been pulverized in their course through the air if they had not been protected by their heat-formed crust. Further, when aëroliths of this species are moistened with a little water, they are completely disaggregated and reduced to extremely fine particles in consequence of the solution of the alkaline salts which perform to them the part of a cement. Under this property, if it had been raining when the Orgueil meteorites fell, on the 14th of May, 1864, or if they had had to pass through a stratum of cloud, they would have been dissolved in their course, and all we should have found of them would have been a little black slime on the ground.

Extra-terrestrial dusts usually reach us under quite different circumstances, and without the intervention of water. The meteoric