426 



NA TURE 



\August 30, 1888 



the surface, and some that fell at Hessle on ice only rebounded 

 without cracking it. 



These bodies, when they fall under such conditions that they 

 can be picked up and examined, are called meteorites. The 

 first thing that strikes one when looking for the first time at 

 these meteorites, is that their general form has the character of 

 being essentially fragmentary, indicating that what we see is the 

 result of a fracture. 



The next point observed is that there is a very great 

 difference between the interior and exterior appearances of 

 these bodies. That this is caused by the heat and friction to 

 which the exterior surface is exposed is proved by what was 

 noticed in the case of a meteorite that fell at Butsura in 1861. 

 Fragments of this stone were picked up three or four miles 

 apart, and, with the exception of one corner, the original 

 meteorite has been built up again by piecing the fragments 

 together. The faces fit perfectly. Important pieces of this 

 meteorite are in the British Museum, and these are all coated 

 with the cru t to which reference has been made. But, on 

 the other hand, another of these fragments not coated fits 

 another also not coated. Hence, to quote Prof. Ma-kelyne, 

 Jt We can assert that this aerolite acouiied, after coming into 



our atmosphere, a .'coriated and blackened surface or incrusta- 

 tion. The first explosion drove the fragments first alluded to 

 asunder, and these became at once incrusted on their broken 

 surfaces ; but others which were separated afterwards, probably 

 on the last of the three explosions, had not sufficient velocity 

 left [the heat being at the same time reduced] to cause their 

 incrustation in the same manner as was the case with the 

 fragments previously severed." l 



The supposition is that the temperature is practically high 

 enough to melt the meteorite, and that its surface as we 

 see it after it has fallen does not in all cases represent the 

 surface exposed to the air during the whole of the flight, but 

 that it represents the last surface. The meteorite may have 

 been twenty limes bigger, but the rest may have been melted off 

 like tallow would be, so that finally there is very little visible 

 effect towards the interior, as the melting is more rapid than 

 the conduction. The thinness of the so called varnish, then, is 

 caused by the air-molecules carrying away the re-ults of fusion 

 as fast as the heat penetrates towards the interior, so leaving 

 01 ly. as a rule, a very thin film behind. 



This crust is usually dull, but sometimes, as in the Strmnern 

 meteorite, bright and shining, like a coating of black varnish. 



Fig. i. — Mazapil Meteoric Iron (f natural s'ze), showing thumb-marks 



^Sorby, 1 en examining with a microscope a thin section of a 

 meteorite, cut perpendicular to the crust, found that it is a true 

 black glass filled with small bubbles, and that the contrast 

 between it and the main mass of the meteorite is as complete as 

 possible, the junction between them being sharply defined, except 

 when portions have been injected a short distance between the 

 crystals. He writes: -- " We thus have a most complete proof of the 

 conclusion that the black cru.-t was due to the true igneous fusion 

 of the surface under conditions which had little or no influence at 

 a greater depth than 1/100 of an inch. In the case of meteorites 

 of different chemicd composition, the black crust has not re- 

 tained a tuie glassy character, and is s mietimes 1/50 of an 

 inch in thickness, consisting of two very distinct layers, the 

 internal showing panicles of iron which have been neither melted 

 nor oxidi/.ed, and the external showing that they have been 

 oxidized and the oxide melted up with the surrounding stony 

 matter. Taking everything into consideration, the microscopical 

 structure of the crust agrees perfectly well with the explanation 

 usually adopted, but rejected by some authors, that it was formed 

 by the fusion of the external surface, and was due to the very 



''On the Structure and Origin of Meteorite;," Natuhk, vol. xv. p. 495. 



rapid heating which takes place when a body moving with 

 planetary velocity rushes into the earth's atmosphere— a heating 

 so rapid that the surface is melted before the heat has time to 

 penetrate beyond a very short distance into the interior of 

 the mass." 



In some cases close under the crust is found a mixture of the 

 minerals troilite, asmanite, and bronzite, of an unaltered light- 

 brown colour, although they turn deep black when raised to a 

 temperature slightly above that at which lead melts. - 



The crust or varnish of the meteorite in many cases contains 

 numerous furrows and ridges, so that it is not equally thick. 

 This effect is caused, as it is supposed, by its motion through 

 the air in a fixed position, the forward, part of the meteorite, in 

 regard to its line of motion, being most liquefied, and tli 

 flowing unequally towards the hinder part. 



A very special study of the results of the passage through the 

 air is a desideratum. Thus, in the case of the Tennessee iron, 

 which fell from a cloudless sky (and which therefore fell with a 

 low velocity?}, the outer surface is elaborately reticulated, edges 



" Lecture Notes," toe. cit. p. 487. 

 Fl.ght, " History cf Meteorite*," p. 





