380 THE POPULAR SCIENCE MONTHLY. 



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 pro- 

 duced 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, vegeta- 

 ble, and animal. These are recognizable by careful examination ; and, 

 after they are all detected, there remain still other dusts, which incon- 

 testably come to us from regions foreign to our globe. The carbona- 

 ceous meteorites of Orgueil furnish us a very interesting prime docu- 

 ment respecting them. These bodies are so friable that they are re- 

 duced 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 

 aeroliths 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 cir- 

 cumstances,, and without the intervention of water. The meteoric 



