376 



NA TURE 



[February i4i 1895 



This depeoded absolutely upon the application of ihermo- 

 dynaraics. and had been pointed out by Helmholtz in the year 

 1845. Sir William Thomson, now Lord Kelvin, also pointed 

 out quite distinctly that the liypothesis of fiery nebulous matter 

 — ^by that meaning nebulous matter hotter ilun the s'ars — 

 was invented before the discovery of thermodynamics ; other- 

 wise, he said, the nebulx would certainly never luve been 

 conceived to have been fiery, i.e. something hotter than the 

 average sur. 



I then went on to show that Lord Kelvin told us how he 

 -onld imagine a condition of nebula- which might ultimately 

 condense into stars without violating the laws of thermo- 

 dynamics, which were ompletely traversed by Laplace's view ; 

 and he referred In a sugjestion that hid been made by Prof. 

 Tait, who supposed that the luminosity of nebula-, and even the 

 spectroscopic appearances which have been observed, might be 

 explained bysuppo^ing that weweredealing with gaseous exhala- 

 tions proceeding from the collisions of meteoric stones ; and he 

 also pointed out that possibly that would not only explain the 

 luminosity of nebula-, but ilie luminosity of comets a-^ well. By 

 the kindness of the Director of this Museum, 1 have some 

 -•specimens of these meteoric stones on the table. I would re- 



might be explained by the fact thai, in consequence of the 

 collisions between these bodies occurring under different con- 

 diiion.s and at ditTerent velocities, there would be very consider- 

 able difterences in ihe temperatures produced in the two cases. 

 Similar conditions might hold (orsiarsindifferent degrees of con- 

 densation. It was also suggested that this new idea might explain 

 the phenomena of variable and new stars, which have always been 

 accounted to be the most extraordinary and mysterious in the 

 whole domain of astronomy ; and, finally, I said the subject was 

 well worthy of study, because it seemed as if many phenomena on 

 the nearest star to us, our own sun, might be really phenomena 

 produced by the fall of meteoric bodies upon that surface which 

 we see, and which we call the photosphere. It is now many 

 years ago since Balfour Stewart and others threw out the idea 

 that the phenomena connected with the formation of sun-spots 

 were really produced by the fall of bodies upon that surface. 

 Other philosophers have preferred the idea that we have to 

 do with eruptions from the interior of the sun ; nothing can be 

 more divergent than the opinions which have been broughi 

 forward as explanations of these appearances. 



But you at once see that, if we assume that (his meteortic 

 action may take place in the solar atmosphere, it need not 



a HERCULIS u-Roup ij) 



■;< CVJ\. ;o;. 



_L 



.\*<v 



Fig. 3. — The photographed jipeclnim of.-! nebula, conli. -uteri with the speclr.-» of stars. 



mind you that the few specimens which I have here have been 

 selected from Ihe magnificent collection upstairs ; if you have a 

 few minutes to spare after this lecture, you cannot do better 

 than go and have a look at them, and you will sec how very 

 various both to the eye and in chemical and physical consfiiulion 

 they are. I.ct me also recommend you to get a little pamphlet 

 (price 2</.) containing a description of the me'eori-e collection 

 in Ihe Natural History Museum, which is one of the finest in 

 the world. 



We thus arrived .-11 the idea that these wonderful ncbul.a- may 

 be explained, apart from any fiery gas ; that we have simply to 

 look lo a meleorilic origin to explain both Ihe appearances and 

 the spectrum. 



After that point had been made, I went on to make another. 

 I had already referred 10 the claMificalion of stars, and I 

 remarked that if one looked at Ihedifferent groups of spectra, it 

 neemed an if a cla»«ificaiion of Ihem, l>a»ed nn thc«e idea», did 

 fit Ihe fact! heller, the existing ones depending on Ihe iinphilo- 

 sophic one of I^aplace. It i< possible, I said, ihat Ihe great 

 tlifference" which had been observed in iheKpccira of comelsand 

 of nebulae, allhough the origin of Ihe light of both was ascribed 

 olhecla-hing together of Lionel in ctifTerent pans of space, 



MO. I.-^ZO vol.. - 1] 



necessarily be a meleorilic action coming from without. Taking 

 our own case, we live in a damp climate, and sometimes the air is 

 dampest when there are no clouds. Clouds are condensations 

 of ilie moisture in the air, and we know that it is not really 

 a question of clouds only ; we may have snow, rain, or hail, and 

 .ill these represent diJTerenl condensations of the damp — or, as 

 we call it, the aqueous — vapour which is ever present in ourair. 

 .-\pply lhat In the sun. What is the air of the sun composed of? 

 Well, ceriainly one important constituent of it is the incan- 

 descent vapour of iron ; we are no longer dealing with a low 

 lemper.aluie and the vapour of water, hut with an atmosphere in 

 the hotter parts of which iron is nn' solid or liquid, but in 

 which the temperature is high enough to keep it in a slate of 

 gas, probably thousands of degrees higher than is arrived at in 

 the Bessemer process. 



We villi assume, then, lhat lhat temperature and that con- 

 dition of atmosphere prevails for 20.000 (it is protubly nearer 

 50,000) miles above the photosphere of the sun. As we get 

 Itirlhcr from the sun, the atmosphere is of course gel'ing cooler, 

 and at a certain distance almve the photosphere Ifie temperature 

 will be so reduced thai the iron vapour iniijlit ))lay the part of 

 our aqueous vapour ; then it condenses and turns into iron snow 



