METEORITES, METEORS, AND SHOOTING-STARS. 743 



can be raised about the origin and behavior of comets. Comets exist 

 in our system, and have their own peculiar development, whatever be 

 our theories about them. It will be enough for my present purpose to 

 assume as probably true the usual hypothesis that they were first con- 

 densed from nebulous matter ; that that matter may have been either 

 the outer portions of the original solar nebula, or matter entirely inde- 

 pendent of our system and scattered through space. 



In either case the comet is generally supposed, and probably must 

 be supposed, to have become aggregated far away from the sun. This 

 aggregation was not into one large body to be afterward broken up by 

 disruption or by solar action. The varieties of location of the cometic 

 orbits seem inexplicable upon any such hypothesis. Separate cen- 

 ters of condensation are to be supposed, but they are not a priori un- 

 reasonable. This is the rule rather than the exception everywhere in 

 Nature. Assume, then, such a separate original condensation of the 

 comet in the cold of space, and that the comet had a very small mass 

 compared with the mass of the planets. Add to this the comet's sub- 

 sequent known history as we are seeing it in the heavens. Have we 

 therein known forces and changes and conditions of such intensity and 

 variety as the internal structure of the meteorites calls for ? 



What that structure is, and to some extent what conditions must 

 have existed at the time and place of its first formation, and during 

 its subsequent transformations, mineralogists rather than astronomers 

 must tell us. For a long time it was accepted without hesitation that 

 these bodies required great heat for their first consolidation. Their 

 resemblance to the earth's volcanic rocks was insisted on by mineralo- 

 gists. Professor J. Lawrence Smith, in 1855, asserted, without reserve, 

 that "they have all been subject to a more or less prolonged igneous 

 action corresponding to that of terrestrial volcanoes." Director Hai- 

 dinger, in 1861, said, " With our present knowledge of natural laws, 

 these characteristically crystalline formations could not possibly have 

 come into existence except under the action of high temperature com- 

 bined with powerful pressure." 



The likeness of these stones to the deeper igneous rocks of the 

 earth, as shown by the experiments of M. Daubree, strengthened this 

 conviction. Mr. Sorby, in 1877, said : "It appears to me that the con- 

 ditions under which meteorites were formed must have been such that 

 the temperature was high enough to fuse stony masses into glass ; the 

 particles could exist independently one of the other in an incandescent 

 atmosphere subject to violent mechanical disturbances ; that the force 

 of gravitation was great enough to collect these fine particles together 

 into solid masses, and that these were in such a situation that they 

 could be metamorphosed, further broken up into fragments, and again 

 collected together." 



Now, if meteorites could come into being only in a heated place, 

 then the body in which they were formed ought, it would seem, to 



