72 



NATURE 



{May 23, 1872 



II. o '2653 gramme of carefully selected substance, evaporated 

 with an excess of ammonium fluoride, left O'ooai gramme 

 residue, cliielly iron oxide. 



This determina'ion denotes the following percentage compo- 

 sition : — 



.Silicic acid .... [9921] 

 Iron oxide, &c. ... o 79 



Besides tlie distinct cleavage parallel to the plane 00 i alieady 

 alluded to, and the other, less distinct, parallel to the planes of 

 the form (10 i ), there seem also to be divisional planes or, rather, 

 surfaces along which the crystals bre.ak up with the greatest 

 facility ; even drying them on blotting-paper proving often suffi- 

 cient to destroy the integrity of specimens that might otherwise 

 seem to promise good results to the goniometer. 



Parlsch, * in his description of the Vienna Collection of Mete- 

 oiites, identifies as a specimen of the Steinbach siderolite a frag- 

 ment with a label, " Native iron, jagged and hackly, with quartz 

 in grains and a yellow fluor-spar" {gi\licgcnes, za/inic/u uiiJ 

 zarkkht gcMichseiies Eiscn iiiit l;oniiclilcin Qiiarz iind i^clblichtem 

 F'litssspatli], 



Breithauptt, in his paper describing the Rittersgriin Siderolite, 

 mikes its chief silicate to be peridot. It is doubtless bronzite. 

 In addition to troilite and schreibersite, he records the presence 

 of "another mineral, the composition of which is not yet deter- 

 mined." 



It should be mentioned that, with a view to test the relative 

 solvent action of alkaline carbonates on quartz and the meteoric 

 silica, weighed portions of each were digested with a ten percent, 

 solution of sodium carbonate for ten hours at ioo° C. under pre- 

 cisely similar conditions. Of the ([uariz 7 '843 per cent, had 

 dissolved, of the Breitenbach silica 9 437 per cent. 



The Shalka A/d,vHlf. 

 An examination of the Shalka meteorite, also detailed in the 

 same memoir, gave as a result that this meteorite consists mainly 

 of a bronzite of the formula Mgj Fej .Si Oj, and is therefore idenii- 

 cal in composition with, though very different in appearance and 

 structure from, that of the meteorite of Manegaum. 



HEAT CENERATEV DY METEORIC STOSES 

 IN TRA VERSING THE A TMOSPHERE 



AS supplementary to my short article on the "M.aximum 

 ■'"*" Velocity of Meteoric Stones Reaching-the Surface of the 

 Earth "(Nature, Sept. 14, iS7i,vol. iv. p. 39S), permit me to add 

 the following observations in relation to the source and the 

 amount of the heat generated by such masses in traversing the 

 atmosphere. 



It is well known that the observations of GUisher, Petit, 

 Daubree, and others, establish the fact that these bodies enter 

 our atmosphere with velocities which are truly planetary. For 

 example, the meteorite of Orgiieil moved with a velocity of at 

 leist 20 kilometres (I2'43 miles) per second, while in other cases 

 velocities have been observed which could n^)t be ob-^erved at less 

 than from 15 to 30, or even 40 mdes per second. The enormous 

 resistance encountered by such bodies in tr.aversing the air {as we 

 have seen in t'le previous article), s,etdily exiinguishes this high 

 velocity, so that tlicy retain but a comparatively moderate \e.o- 

 city on reaching tlie surface of the earth. 



Now, the '■Dynamical Theory of Heat" assures us that 

 whatever energy is lost by the moving body is transformed imo 

 heat, which i, either retained by the body or is communicated to 

 the air. The amount of heat thus generated can be estimated, 

 p.ovided the diminution of velocity and the mass of the meteoric 

 stone are known. More than 23 years ago Dr. Joule pjinted 

 out this transform.ation of energy into heat as the true cause of 

 the ignition of thesi extra-terrcslrial masses in traversing the air 

 (I'uic Phil. Mag., 3rd Series, vol. xxxii., p. 349, May 1S4S). 



According to Joule's experiments, one French unit of heat 

 = 4235424 kilogrammetres or units of energy. Hence, one 



French unit of energy = ■ = O'oo236l04 French units of 



423-542 + 

 heat. If, the efore. 



1 k, k. Hof-Mii 

 tilling, xxi. p 



ilicn-KiiljineUe. Wic 



V = velocity of stone upon entering the atmosphere, 

 t'l = ,, ,, ,, when rear the earth's surface, ' 



g = acceleration produced by gravity, 

 w — weight of moving stone in kilogrammes : 

 Then if f, '\, and g are taken in metres per second, we have. 



Loss of energy = 10 . ^' . 



This loss of energy transformed into heat = 



o'00236iO4 X w . !— French units of heat. 



2g 



Assuming ,i,'- = 9'So942 metres per second, we have, 



French units of heat generated = o •01x3120345 x w x (r/--!,-). 

 This gives the total amount of he.at evolved by the loss of energy. 



Now, if we knew the distribution of the heat thus generated, 

 it would be easy to estimate the increase of temperature which it 

 would occasion in the matter to which it is imparted. If we 

 assume all of the heat evolved to be retained by the moving 

 body, it is evident that the increase of temperature («/- — z\- being 

 the same) is inversely proportional to the product of the weight 

 into the specific heat of the moving mass. 



Hence, if c = specific heat of stone in relation to water as 

 unity, we have. 



Increase of temperature in C = 0'OOOI20345 x '—^ -— ; or 



Increase of ,, ,, = o •000120345 x {v- — v^-) x -. [a) 



Under the assumption that all of the heat generated is retained 

 by the moving mass, it is obvious that w is eliminated ; for, 

 while the heat evolved varies as jc, the mass to be heated also 

 varies as 10 ; so that the weight is without influence, in so far as 

 increase of temperature is concerned. 



In order to apply this formula to any given case, we must 

 know c, -'j. and c in relation to the moving stone. Perhaps we 

 should not be over-estimating ;■ by assuming it to ba 30 kilo- 

 metres per second, or nearly equal to the velocity of the earth in 

 its orbit. In like manner we should certainly not be under- 

 estimating r'l by assuming it equal to 500 metres per secoLd. 

 .Sulistituling these values in formula {<i), and we have. 



Increase of temp, in C° = o^oooi20345 (30000- - 500") x _ ; or 



Increase of ,, ,, = 10S530 x 



Finiliy, in the absence of experimental results in regard to the 

 specitic hfat of meteoric stones, we are certainly not under- 

 estimating c- by assuming it equal to o'22. 



Substituting this value of c in the last equation, and we have. 



Increase of temp, in C° = 492, 184". 



Of course, by far the larger portion of the heat generated by 

 the loss of energy of the moving stone would be imparted to the 

 air along i^s trajectory ; but assuming that only ^Jjth pait of 

 it is retained by the stone, it would be more than sufficient to 

 account for the phenomena of fusion and detonation which fre- 

 quently accompany the transit of such bodies through our 

 atmosphere. 



In the case of small masses, it is clear that their high velocities 

 would be more rapidly extinguished by the resistance of the air 

 than is the case with large masses. In the small mass the trans- 

 formation of energy into heat being accomplished in a shorter 

 time, a greater amount of the evolved heit would be retained 

 by the stone than in the large mass whose velocity is more 

 gradually checked by the resisting medium. 



Htnce, when the smaller masses plunge into the upper atmo- 

 sphere, the matter may be volatiliseii or utterly dissipated by the 

 intensity of the suddenly-evolved heat. In this minutely- 

 divided condition the material of the stones would float about in 

 the atmosphere, and ultimately reach the surface of the earth in 

 the form of meteoric dust. 



It is well known that the observations of ISenzenbe^g, 

 Quetelet, Herrick, Newton, and others, assign to the so-called 

 "falling stars" velocities equal to, if not surpassing, the velocities 

 of meteoric stones. According to the foiegoing suggestion, these 

 may be nothing more than small meteoric stones which are volati- 

 lised in the upper regions of the atmosphere long before reaching 

 the surface of the earth. 



Thu-, the phenomena of the occasion.al fall of meteoric stones 

 and the almost incessant appearance of the falling stars which 

 nightly furrow the celestial vault, may be correlated with the 



