yan. 30, 1890] 



NATURE 



307 



structure, doubly refractive power, and, in fact, is optically 

 orientated like the inclosed silicate. It appears to be produced 

 by some agent acting from without, perhaps heat in conjunction 

 with a reducing gas. The agent has not caused friction, but a 

 slight modification of the texture of the surface. 



Indications afforded by Crystalline Strtuture. 



The mixed minerals of meteorites have been subjected to 

 microscopic examination by Sorby^ and Rose," and both have 

 found that the crystals differ in some essential particulars from 

 those of volcanic rocks. 



Sorby long since showed that when crystals are formed by 

 deposition from water or from a mass of melted rock, they often 

 catch up portions of this water or melted stone which can be 

 seen as cavities containing fluid or glass. Crystalline minerals 

 formed by purely aqueous or by purely igneous processes 

 can thus be distinguished. One of the most common of the 

 minerals in meteorites is olivine, and when met with in volcanic 

 lavas this mineral usually contains only a few and small glass- 

 cavities in comparison with those seen in such minerals as augite. 

 The crystals in meteorites are generally only small, and thus 

 the difficulty of the question is considerably increased. However, 

 by careful examination with high magnifying power, Sorby 

 found well-marked glass-cavities, with perfectly fixed bubbles, 

 the inclosed glass being sometimes of brown colour and having 

 deposited crystals. On the contrary he was never able to detect 

 any trace of fluid-cavities, with moving bubbles, and therefore he 

 holds it very probable, if not absolutely certain, that the crys- 

 talline minerals in meteorites were chiefly formed by an igneous 

 process, like that which has produced lava, and analogous 

 volcanic rocks. 



Passing from the structure of the individual crystals to that of 

 the aggregate, Sorby points out that in some cases we have a 

 structure in every respect analogous to that of erupted lavas, 

 though even then there are very curious differences in detail. 



The results of the observations of the kinds of crystallization 

 noted in meteorites by many eminent authorities go to show that 

 it took place hastily. Thus Brezina, after making a complete 

 study of the Vienna collection, comes to the conclusion that 

 the structural features of meteorites are the result of a hasty 

 crystallization. 



Again, it is the opinion of several high authorities that the 

 crystallization did not necessarily take place under conditions of 

 high temperature. 



M. Daubree's opinion is thus expressed : — ^ 



•'It is extremely remarkable that, in spite of their great 

 tendency to a perfectly distinct crystallization, the silicate com- 

 binations which make up the meteorites are there only in the 

 condition of very small crystals, all jumbled together as if they 

 had not passed through fusion. If we may look about us for 

 something analogous, we should say that, instead of calling to 

 mind the long needles of ice which liquid water forms as it 

 freezes, the fine-grained texture of meteorites resembles rather 

 that of hoar frost, and that of snow, which is due, as is known, to 

 the immediate passage of the atmospheric vapour of water into 

 the solid state," 



This possibility of the absence of high temperature is thus 

 further insisted upon by Prof. Newton : — * 



" The meteorites resemble the lavas and slags of the earth. 

 These are formed in the absence of water, and with a limited 

 supply of oxygen, and heat is present in the process. But is 

 heat necessary ? Some crystallizations do take place in the cold ; 

 some are direct changes from gaseous to solid forms. We cannot 

 in the laboratory reproduce all the conditions of crystallization 

 in the cold of space. We cannot easily determine whether the 

 mere absence of oxygen will not account fully for the slag-like 

 character of the meteoric minerals. Wherever crystallization can 

 take place at all, if there is present silicon and magnesium and 

 iron and nickel, with a limited supply of oxygen, their silicates 

 ought to be expected in abundance, and the iron and nickel in 

 their metallic forms. Except for the heat, the process should be 

 analogous to that of the reduction of iron in the Bessemer cupola, 

 when the limited supply of oxygen combines with the carbon, 

 and leaves the iron free." 



Should this view be subsequently confirmed, all early ideas 

 touching the formation of meteorites will require to be modified. 

 Thus, in 1855, Prof. Lawrence Smith stated : "They have all 

 been subject to a more or less prolonged igneous action corre- 



' Proc. R.S.. January 1864. * Berlin Acad. Trans. 



3 Quoted by Newton, Nature, vol. xxxiv. p. 535. 4 Nature, loc. cit. 



sponding to that of terrestrial volcanoes." Haidinger, in 1861^ 

 not only declared for high temperature, but for high pressure. 



Obviously, these views, which were based more upon the ana- 

 logues of some of the meteoriteswith volcanic basic rocks than upon^ 

 the actual character of the crystallization, suggested the forma- 

 tion of large masses ; and the ideas that comets were solid bodies 

 and that meteorites were fragments of comets or planets were 

 both based upon these views,^ and the higher the temperature 

 required and the slower the crystallization, the larger in imagina- 

 tion did these possible birthplaces of the meteorites become. 



If neither much time nor heat be required to produce the crys- 

 tallization observed, then, with Prof Newton, we can suppose 

 " a mass containing silicon, magnesium, iron, nickel, a limited' 

 supply of oxygen, and small quantities of other elements, all in 

 their primordial or nebulous state (whatever that may be), segre- 

 gated somewhere in the cold of space. As the materials con- 

 solidate and crystallize, the oxygen is appropriated by the silicon 

 and magnesium, and the iron and nickel are deposited in metallic 

 form. Possibly the heat developed may, before it is radiated 

 into space, modify and transform the substance. The final result 

 is a rocky mass (or possibly several adjacent masses) which sooner 

 or later is, no doubt, cooled down throughout to the temperature 

 of space." 



We shall see subsequently that there are many known causes 

 in operation which will provide us with just such a mixed mass 

 of vapours as Prof. Newton requires, and it is at once obvious 

 that, not only is the generic separation into iron and stones thus 

 accounted for, but the special form of crystallization observed in 

 stones and the special chondritic structure observed both in irons 

 and stones would all arise from the same cause. 



Evidences of Heating and Action of Violent Forces at Different 

 Times. 



The peculiarities in the mineralogical structure of the meteorites 

 are probably in part due to the successive heatings and coolings 

 to which they were subjected with each approach of the comet to 

 the sun, and partly, perhaps, to the heat of combination of oxygen 

 and silicon. They were most probably formed in a limited supply 

 of oxygen, so that the elements possessing greatest affinity for 

 that element were the first to form compounds, leaving iron and 

 nickel in the metallic or uncombined state. 



Some meteoric stones from examination seem to have been 

 heated to a high temperature right through their mass. Such 

 cases as Orvinio, Chantonnay, Juvenas, and Weston show signs 

 that fragments are cemented together with a material of the 

 same substance as themselves. Again we have indications of 

 chemical changes, the chondroi in some stones being found to- 

 be surrounded by spherical and concentric aggregations of 

 minute particles of nickel, due, as is supposed, to the reducing 

 action of hydrogen at a high temperature. 



Some meteorites are merely breccias, consisting of fragments, 

 the dibris of pre-existing meteorites, or of the original mass 

 tremcndou-ly shattered, and subsequently cemented together. 



In this connection Sorby writes : — 



" It would therefore appear that, after the material of the 

 meteorites was melted, a considerable portion was broken up 

 into small fragments, subsequently collected together, and more 

 or less consolidated by mechanical and chemical actions, amongst 

 which must be classed a segregation of iron, either in the metallic 

 state or in combination with other substances. Apparently this 

 breaking up occurred in some cases when the melted matter had 

 become crystalline, but in others the forms of the particles lead 

 me to conclude that it was broken up into detached globules^ 

 whilst still melted (Mezd-Madaras, Pamellee). This seems to- 

 have been the origin of some of the round grains met with in 

 meteorites ; for they occasionally still contain a considerable 

 amount of glass, and the crystals which have been formed in it 

 are arranged in groups, radiating from one or more points on the 

 external surface, in such a manner as to indicate that they were 

 developed after the fragments had acquired their present 

 spheroidal shape (Aussun, &c.). In this they differ most cha- 

 racteristically from the general type of concretionary globule* 

 found in terrestrial rocks, in which they radiate from the centre ; 

 the only case that I know at all analogous being that of certain 

 Oolitic grains in the Kelloways rock at Scarborough, which have 

 undei^one a secondary crystallization." " 



Mr. Sorby remarks : "A most careful study of their 

 microscopical structure leads me to conclude that their con- 



' See Newton, Natlre, vol. xxxiv. p. S34- 



- "Microscopical Structure of Meteorites," Proc.R.S., June 16, 1864. 



