458 



NA TURE 



[Sept. 6, 1888 



pounds of carbon in the meteorite which require a higher 

 temperature to bring them out, but which come out when that 

 higher temperature is employed. The carbonaceous structure of 

 some meteorites has already been determined on other grounds. 



If we carry the heating a little further still, and instead of 

 leaving the particles relatively cold and dark while the current is 

 passing we apply a higher temperature outside the tube by 

 means of the Bunsen burner, then we get the luminous vapours of 

 some constituents of the meteorite added to the spectra of 

 hydrogen and carbon. 



What luminous vapours do we get first, and which last? The 

 experiment is a very interesting one, and may certainly be 

 carried on in a tube such as that described until a pretty con- 

 siderable development of the spectrum is obtained. The first 

 substance which makes itself visible obviously after the hydrogen 

 and carbon when particles of a meteorite are treated in this way 

 is magnesium derived from the olivine, that substance which 

 exists in the greatest quantity in the stones, and in the schreiber- 

 site, which exists in the irons. 



From such a method of research as this we can pass to one in 

 which, by means of the oxy-coal-gas flame, we can determine 

 the spectrum of any vapour given off, provided any vapour is 

 given off, at a still higher temperature. That work has been 

 done, and the main result is that in the case of an " iron," the 

 first substance to make its appearance is manganese, and the next 

 substance to make itself obvious is iron. 



Here a very important remark must be made. The substance 

 which will give us the predominant spectrum at lowest tempera- 

 ture must be that substance the volatility of which at that 

 temperature is greatest. If, however complicated the chemical 

 constitution of one of these meteorites may be, there is one 

 substance which volatilizes out of it more readily than another 

 at a low temperature, that substance will be the first to give us 

 its characteristic spectrum at that temperature — and in fact we 

 may get the spectrum of that substance alone, although its per- 

 centage in the meteorite may be extremely small. It is therefore 

 an important result to find that in meteorites in which the 

 quantity of iron is very considerable it is always the manganese 

 that makes itself visible first, because its volatility is greater than 

 that of iron. The point to bear in mind is that when we pass to 

 the temperature of the oxy-coal-gas flame we get predominant 

 evidence of the existence of manganese, and afterwards of iron. 



Many diagrams of observations made in this way have been 

 constructed of the oxy-coal-gas flame of meteorites and of olivine, 

 and not only the flame but the "glow," — glow being the name 

 given to the luminosity produced in the tube under the conditions 

 stated. There are some points of similarity, and other points of 

 difference. One of the results which is most constant is a line at 

 500 on the wave-length scale which appears to run through all the 

 observations until we come to deal with such meteorites as the 

 Limerick and Nejed. On the other hand some lines and flutings 

 do not make their appearance generally. 



If we wish to extend our inquiry into the function of a still 

 higher temperature we can use the electric arc ; that also has 

 been done. For this purpose specimens of iron meteorites have 

 been cut into poles, the spectra of which have been observed 

 and photographed, so that the vapours produced have been 

 the vapours of the pure iron meteorites ; that is to say, a small 

 portion of a meteorite lias not been placed in an impure carbon 

 pole, so that the impurities of the carbon would be observed 

 and photographed with the pure vapours of the meteorites. In 

 addition to this method — in the case of the stony meteorites — the 

 lower pole after its spectrum has been well studied has been 

 utilized in this way : the upper pole remaining constant as an 

 iron pole, pretty big particles of various stony meteorites have 

 been inserted into the lower pole, and the added result has been re- 

 corded. Further, composite photographs of the spectra of many 

 meteorites have been obtained. Half a dozen different stony 

 meteorites have been rendered incandescent by their insertion 

 into the lower pole during the exposure of a single photographic 

 plate. 



It is pretty obvious that if we can get detailed information on 

 such points as these, and provided there are meteorites in space 

 at the temperatures at which we are able to determine their 

 spectra in the laboratory, such data should be of extreme value, 

 for at present we know of no reason why the spectra should 

 differ according to locality. 



J. Norman Lockyer. 



( To be continued. ) 



MOLECULAR P//YS/CS : AN ATTEMPT AT A 

 COMPREHENSIVE DYNAMICAL TREAT- 

 MENT OF PHYSICAL AND CHEMICAL 

 FORCES. 1 



II. 



§ 6. Double Refraction. 



A CCORDING to the theories both of Fresnel and of Neu- 

 ■^ mann, double refraction is explained on the assumption 

 that the elasticity of the ether in crystals which exhibit this 

 phenomenon is different in different directions. The elasticity 

 is proportional to the square of the velocity of propagation, 

 and if a, b, c are the ratios of the elasticities, parallel to 

 the principal axes of the crystal, of the ether within it to its 

 density, the velocity in any direction o, 13, 7 will be given by the 

 equation — 



v- = a 1 cos 2 o + b~ cos 2 ]3-f c ! cos 2 7 . . . . (18) 



According to the author's theory, the elasticity of the ether is 

 the same in every direction, so that any difference in the velo- 

 cities of propagation in different directions must be due to the 

 mutual action between the ether and the molecules of the crystal 

 being a function of the direction, and therefore the values of the 

 quantities ci for the molecules of the crystal, and hence also the 

 value of fi, must depend on the direction. 



Assuming, for simplicity, that the molecules have a single 

 shell only, it follows from (8) and (9) that — ■ 



2= J_ = p _ ^T2 



v- I 



I 



1 + 



T" 



- ( 1 - c« 



lia -rT-Ti +c Ti ^ R i V 



(19) 



where /Cj 2 = m 1 /(c 1 + c s ) and R a = «j/*i 2 fci + c„). 



Let the values of Ci and /* for a second direction be a" L and /j. 1 , 

 then 



^ = ~ 



I 



r* 



( "' + " , >C7T?-' P ) 



(20) 



Now, as Thomson has pointed out, the dispersion accompany- 

 ing double refraction is of very small amount, so that the 



difference ,u 2 - fi 1 ' must be sensibly independent of T. 



If T were less than k, (j? - (j. 1 - would, from (12), be propor- 

 tional to T J . It must therefore be assumed that the critical 

 period is at the extreme blue end of the spectrum, which will 

 give T greater than k x for all the rays. Then from (12a) — 



ft 



1" c \"»h 



l{c x + c 2 f / {Ci i + ^ 1)2 

 V 1 l c l+ c 2 + c^ + c.OT 



2 l 2 * 



+ C J , C -l fn ±- + (21) 



(c t + c 2 )*> (q 1 + ^) 3 /T» 



In order that the coefficient of T 2 may be small, c 1 and ej 

 must be small and nearly equal. The other terms of the series 

 will then be also very small, especially if T is large in com- 

 parison with m lt and the series may, to a first approximation, 

 be replaced by its constant term. 



Now let it be assumed that the manner in which c x and <r 2 

 depend on the direction a, fl, 7, is determined by an equation of 

 the form — 



(22) 



I — L_ \ — d cos 2 « + Co cos 2 y3 + C 3 cos 2 7 

 Vj + c%' 



Then from (19) and (12a) — 



v = — — ( - _ — 1 C x ) cos- a + ( — - — x Co ) cos- £ 

 pr \ P p ' \ p P ' / 



+ ( * C, ) cos- 7, 



\p p V 



an equation of the same form as (18), and which therefore gives 

 a wave-surface identical with Fresnel's. It must, of course, be 



1 A Paper read before the Physico-Economic Society of Kon'gsberg, by 

 Prof. F. Lindemann, on April 5, i838. Continued from p. 407. 



