August 30, 1888J 



NA TURE 



427 



of thin laminae of metal inclined at angles of 60' traversing it. 

 Hence no fusion of the superficial layer took place. 1 



Another peculiarity of the surface is that it is generally covered 

 with small depressions called " thumb-marks," as they have been 

 likened to the impressions that one makes when pressing some 

 such substance as putty with one's lingers. The cause of these 

 thumb-marks is unknown, but they have been found to bear a 

 close resemblance to marks which have been noticed on grains 

 of gunpowder blown out on firing large guns. 



A possible cause of these pittings 'is thus suggested by Prof. 

 Maskelyne: — "'The aerolite comes into our atmosphere from 

 regions in which the temperature— ' the cold .of space '—may 

 range as low as 140' C. below zero ; and though the mass, from 

 the absorption of solar heat, would possess a temperature much 

 above this, it would nevertheless be intensely cold, and conse- 

 quently more brittle than at ordinary temperatures ; and hence, 

 on its entering our atmosphere, the heat it instantaneously ac- 

 quires on its outer portion expands this, and tends to tear it 

 away, so as to dissever the exterior from the interior, which 

 continues to be relatively contracted by the intensity of the cold 

 which the aerolite brings with it from space. The' consequence 

 is, first, that little bits of the stone spring out all over it, leaving 

 those curious little holes or pit-marks which are characteristic of 

 a meteorite ; and every now and then, as the heat penetrates, 

 larger masses split away, of which interesting evidence is afforded 

 by the meteorite, for instance, that fell at Butsura on May 12 

 1861." ' ' 



On this it may be remarked that the pittings are common to 

 irons and stones, while the above explanation only applies to 

 stones. 



It is not a little worthy of notice that the pitting does not 

 always appear on all the surfaces. In the ease of a meteorite 

 which fell in Kentucky in 1877, one portion of it is very extensively 

 and regularly pitted, while the rest is comparatively smooth. 

 The crust is dull black, and is as perfect as when the stone 

 fell. There was a fresh broken spot of two or three square 

 centimetres, which was evidently made prior to the fall, for a 

 few small specks of the melted matter adhered to the surface.- 



These meteorites, which we can thus examine, are in all prob- 

 ability, for the most part, remnants of larger bodies which had 

 enough substance in them to stand the wear and tear of getting 

 j through our atmosphere. 



The fragments picked up even from the most extensive 

 falls have appeared to those who have witnessed or who have 

 subsequently studied the phenomena to be out of all proportion 

 small to the violence and magnitude of the explosive and 

 luminous effects observed. 



The origin of the concomitant phenomena so universally 

 recorded is not far to seek. 



Supposing a meteorite passing towards the earth through the 

 atmosphere, what sort of effects are we to expect to find? It 

 passes, as we have already seen, very rapidly into the earth's 

 atmosphere, which consists of molecules with a certain mean 

 free path, and the temperature and pressure of which depend 

 upon the encounters between these molecules. 



When we come to condder the general velocity of movement 

 ■"these molecules, we find that the big molecule.' the meteorite, 

 is travelling towards the earth about fifty times faster. The 

 Result is that there is a tremendous crowding of air, so to speak, 

 in front of the meteorite, a tremendous pressure and therefore a 

 tremendous temperature brought about by its passage. There is 

 a partial vacuum behind which subsequently has to lie filled up 

 by the transit of the molecules round the meteorite itself from 

 the front part to the back. 



We have therefore conditions for producing most violent 

 action upon the meteorite, both by pressure and temperature ; 

 it may be crushed by the pressure to which it is subjected, it may- 

 be melted by the heat produced by the circulation' of the mole- 

 cules rushing past it. We may therefore have violent incan- 

 e and explosion, and as we have the air molecules rushing 

 violently fro 11 front to rear we shall have almost the noise 

 of a thunderstorm added to the sudden luminosity resembling 

 lightning. 



The observers of actual falls have heard other special noises, 

 due, not to the explosion itself, but to the rapid passage of the 

 meteorites through the air, from the " ping" of a rifle bullet to 

 the hum of a locomotive, sounds which have been likened to the 

 tearing of linen, the lowing of cattle, the Happing of wings. 



We can best study the differences in the structure of 

 meteorites by preparing a polished section. In some cases this 

 1 Flight, op. cit. p. 108. 2 Ibid. p. 200. 



has a distinctly metallic look. We find, in fact, a metallic frag- 

 ment composed almost entirely of iron, but with a certain amount 

 of nickel. 



The nickel in the iron meteorites causes them to have a whitish 

 appearance, and it is in this way that they have been mistaken 

 for silver when found, the nickel preventing the outer surfaces 

 from rusting as is the case with an ordinary iron. 



By taking a polished section, and exposing it to the action 

 of an acid or bromine, we obtain what have been called the 

 "figures of Widmansiatten." These figures are more or less 

 complicated, and remarkable for their extreme regularity. They 

 are due to the inequality of the action of the acid on the various 

 constin: .its of the polished surface ; these being various alloys of 

 iron and nickel. 



In other specimens the characteristic is that the metal, 

 instead of being continuous as in those previously referred to, 

 appears to have existed once as a spongy paste, and to have 

 included fragments of stony matter, so that in the section, in- 

 stead o( getting the pure metallic lustre all along, we only get 

 it here and there. We pass from metal to metal ///is stone. 



Iq yet other specimens we get another generic case repre- 

 sented in which the stone is the main point and the metal the 

 exception, the metal appearing as excessively small granule- 

 thai in the final term of the series we come to almost pure stone 

 with no iron to speak of. 



In :he case of the stones, not only does the meteorite itself 

 give the idea of a fragment, as in the case of the irons, but the 

 internal structure of many of them shows that the whole 

 meteorite is composed of fragments, giving the characteristics of 

 a brecciated rock made up of pieces cemented together. 



Fig 2. — Stction of Mazapil Meteoric Iron (natural size), showing 

 Wid.nanstiittun figures. 



Further, these constituent particles, as pointed out by Sorby, 

 are often themselves mere fragments, although the entire body 

 before being broken may originally have been only one fortieth 

 or one-fiftieth of an inch in diameter. 



On examining thin sections of stony meteorites by means of 

 polarized light, they are found to be crystallized throughout, the 

 interference tints colouring the different crystals of which the 

 sections are composed, thus showing the crystalline character of 

 the whole. The stony part of both siderolites and aerolites is 

 almost entirely crystalline, and presents a peculiar " ch ndritic" 

 structure, which make meteorites differ from ordinary terrestrial 

 rocks ; the loose grains in these are found to be mote or less 

 aggregated in little spherules, and of similar mineral to those 

 which inclose them. 



These spherules, or chondroi — their sizes varying very con- 

 siderably, some of which can be seen only under a microscope, 

 while others are as large as a cherry — are found embedded in a 

 matrix, made up, as it appears, of minute splinters such as 

 would result from the disintegration of other chondroi. 



While the chondroi in terrestrial rocks such as perlite, 

 obsidian, pitchstone, and in many diorites, are radiate- hbrous, 

 those occurring in meteorites are but rarely so, and the arrange- 

 ment of the fibres within the spherule is eccentric. While the 

 meteoritic chondroi als > consist of the same ingredients as the 

 matrix, and often differ from it only in being more coarsely 

 g-anular, the chondroi of terresinal rocks are differently 

 constituted from the matrix. 1 



The weight of meteorites varies very considerably, ranging 



from tons to very small specimens. It not only depends on their 



volume but on their chemical composition, as so i,e of '.he stony 



oneshavea low density while oiheis are nearly pure metal. 



The largest meteorites of which mention is made are those 



1 Ibid. p. 141. 



