PRE-TERRESTRIAL HISTORY OF METEORITES 629 



were arranged in order of their densities and that it had at some 

 time a hot interior and cold exterior. 



The first is drawn from our knowledge of the crystallography 

 of iron. According to modern metallographists, iron occurs 

 in three allotropic modifications known as alpha, beta, and 

 gamma irons. When heated to a temperature not higher than 

 700° C, iron remains in what is known as the alpha state ; from 

 700 to 860° C. it assumes the beta state, and from 860° C. to 

 the melting point, the gamma state. In cooling, from the melt- 

 ing point, for instance, the iron does not necessarily return 

 through these modifications, but remains in the state which it 

 assumed at the highest temperature.^ Now gamma iron crys- 

 tallizes in octahedrons, while alpha and beta iron crystallize in 

 cubes. The majority of meteoritic iron is plainly octahedral in 

 structure. It is hard to escape the conclusion, therefore, that it 

 has been heated to a temperature at least as high as 860° C, 

 and, further, that the cubic irons, some of which occur among 

 meteorites, have been subjected to a somewhat lower degree of 

 heat. The latter, it is true, must be cooled and reheated to a 

 somewhat lower temperature than at first to have their structure 

 accounted for on this theory. But this could quite reasonably 

 occur, and their relative quantity is so small as to make them of 

 minor importance. 



A second corroborative fact is that the carbonaceous 

 meteorites are of exceptionally low specific gravity. Now the 

 carbonaceous meteorites are those which contain hydrocarbons 

 which could not exist under any high degree of heat. Such 

 meteorites could not have experienced any sensible heating 

 subsequent at least to the formation of these hydrocarbons. 

 But the low density of these meteorites would place them, 

 according to the hypothesis, on .the outer surface of the sphe- 

 roid, where, after the first solidification from cooling, little 

 further heating would be encountered. 



A third corroborative fact is found in the existence of 

 diamonds in the iron meteorite of Cation Diablo, a meteorite 



'F. Osmond, The Metallogj-aphist, July, 1900. 



