3o6 



NA TURE 



\yan. 30, 1890 



is to establish the connection between comets and meteorites, as 

 even the supporters of the theory do not claim that the comets 

 are parts of a broken planet. Then, again, it is only an assumption 

 that such a planet ever existed, and it is difficult to understand 

 how a broken planet should so far disobey the law of gravity as 

 to divide itself into small scattered fragments. 



The real parentage of those meteorites which fall on our earth 

 is, therefore, probably cometic, for the association of comets, 

 meteorites, and shooting-stars can no longer be denied, and it 

 is an observed fact that comets do break up. 



The discovery of Schiaparelli (1866), and his view that the 

 head of a comet was the largest meteorite in a swarm, of course, 

 put these origins of some meteorites, at all events, out of the 

 question. 



Reichenbach (1858) did not consider that the head of a comet 

 was a large meteorite, but a swarm of small ones, and the large 

 meteorites he considered to be built up in some way out of the 

 smaller ones bi-ought into our system by comets. If this view be 

 subsequently confirmed, since we now know that, as suggested by 

 Schiaparelli, comets are nebulous shreds, brought into our 

 system by solar or planetary attraction, it follows that in the 

 nebulae also we may be only dealing with excessively small 

 masses. 



If meteorites, in the restricted sense of the term above referred 

 to, do not exist sporadically in external space, they must be 

 manufactured in our system, and two tests should be open to us : 

 (i) no meteorites should reach us from outer space ; and (2) they 

 should bear traces of the process by which they have been built 

 up from cometary materials. 



If we can establish this, then we imagine a gradual progression 

 in the size of meteoritic masses from regions where they are so 

 small that luminous collisions are all but impossible, to those 

 regions nearest to a cooling sun, like our own, where there 

 has been the richest supply of cometary material, furnished at 

 successive perihelion passages for the longest time. 



With regard to (i), we have the facts that it is only very rarely 

 meteorites fall in displays of shooting-stars, and that when the 

 earth has passed near a comet no increase in the avei-age number 

 of meteorites has been noticed. 



The most important piece of evidence on this point, however, 

 has been recently furnished by Prof. Newton, who, from a com- 

 plete discussion of the data extant from all known falls, has 

 come to the conclusion that all the meteorites now in our 

 collections have come from a single ring of bodies circulating 

 round the sun. 



We next come to (2). The most important point to consider 

 here, in the first place, is the very special structure of meteorites. 



Thumb Markings. 

 Regarding the origin of the remarkable pittings of the 

 surfaces of aerolites and aerosiderites, an opinion was lately 

 expressed and advocated by Daubree,^ that in their flight 

 through the air they undergo erosion and excavation by joint 

 effects of combustion and fusion, assisted mainly by air vortices 

 attacking most violently certain portions of their surface. An 

 important paper on this subject by Prof. Maskelyne was published 

 immediately afterwards in the Philosophical Magazine (of August 

 1876). It is true that pittings identical in appearance with those 

 of meteorites are found on the surfaces of certain large grains of 

 powder blown unconsumed from the mouths of the large modern 

 rifled ordnance (excellent specimens of this kind received from 

 Prof. Abel and Major Noble having been shown by Prof. 

 Maskelyne to M. Daubree in the summer of 1875) 5 but two 

 important grounds for exception, in regard to this explanation, 

 are pointed out by Prof. Maskelyne, which must not be over- 

 looked. The closest examination of the molten glaze with which, 

 like other parts of these surfaces, the pittings or depressions of 

 meteorites are coated over, shows no indications of vorticose 

 action of the air, although stream-lines of the glaze from front 

 to rear are of frequent and conspicuous occurrence. The process 

 of atmospheric combination, or combustion, is also rare, if not 

 entirely absent, during the period of most intense operation of 

 the heat, as is shown by particles of metallic iron which are 

 occasionally found embedded in the glaze, and even by cases 

 where the highly oxidizable mineral oldhamite (calcium sulphide), 

 occurring in spherules in the Bustee meteorite, is glazed over 

 equally with the augite without offering any signs of combustion 

 or of the production of cavities where they are exposed. 



' Coniptes rendus, April 24, 1876. See "Report on Observations of 

 tuminovis Meteors for the year 1875-76," p. 167. 



Cho)idritic Structure. 



We have spherules of iron, like small shot of different sizes, in 

 the stones. 



These spherules, or chondroi, as they are sometimes called, 

 vary very considerably in size ; some reach the size of a cherry, 

 while others are so small that they can only be seen by the aid 

 of atmicroscope. 



Chondroi in Soko-Banya meteorite (magnified 10 diameters) 



Chondroi in Mocs meteorite (magnified 10 diameters) 



By examining sections of chondritic stony meteorites we find 

 that they consist sometimes almost entirely of spherules. The 

 Parnellee aerolite affords us a very good instance of this, the 

 most varied groups of spherules being seen collected together in 

 one section. These spherules are sometimes encased in small 

 shells of nickeliferous iron, or sometimes in addition with a kind 

 of pyrites, a sulphide of iron termed troilite (FeS), peculiar to 

 meteorites. 



vSome chondroi have round depressions which point to plas- 

 ticity during contact, as if the spherules which form the splintered 

 fragments had acquired their form during the act of rubbing. 

 Others, again, have projections of a rounded form, or an almost 

 pointed end.^ 



Our terrestrial rocks contain no structure identical with that 

 chondritic structure so peculiar to meteorites, and the characters 

 of the spherules are found to be quite different from those in 

 either perlite or obsidian. 



Tschermak ^ directs attention to the peculiarities observed in 

 several chondritic meteorites. The first is the occurrence of a 

 crust over the surface of the bronzite spherules, possessing fibrous 

 structure. This crust is thin, and is distinguished from the 

 inclosed material by its paler colour ; it has the same fibrous 



' Flight, " History of Meteorites," p. 207. 



2 Quoted from " Report of Observations of Luminous Meteors during the 

 Year 1877-78," p. 107. 



