596 



NA TURK 



\Ocl. 20, 1 88 1 



mind to such a view, aid ihty led him to ado| t another, 

 that the heat w as due to the imp;:ct of meteoric bodies falUiig 

 into the sun. Tlie surroundings of the sun may Ije considered 

 to ci nsit of a vast nuo-bir of meteoric bodies similar to the 

 shontins; ^tars which we see when they come across the eartli's 

 atmo phere. An asseniblage of such bo' ies reflecting in a 

 measure the light of the Min may posibly con-titute the zi diacal 

 light. Now it the. e todies are continually falling into ihe sun 

 their impact will produce an enormous quantity of heat. I 

 should mention that this idea had been thrown cut previoitsly 

 by Waterton, but Sir Wilham Thomson made an important 

 change in it by supposing that instead of being dependent on 

 mete ric bodies casually falling into the sun from the stellar 

 spaces, there is a supply of such bi dies circulating round the 

 sun and gradually falling into it. He showed that the heat 

 produced by sUL-h impacts wiuld enormously surpass the heat of 

 coml ustion of the most comtutible sub-tances we know on 

 e.arih. This theory attriLu'es the heat of the sun to something 

 outside itself; what I may call, incontrnd^stinctirn tothat, primi- 

 tive hea', attributes it to v\hat is inside the sun, to the body itself. 

 Accordi g to the meteoric theory the seat of the most intense 

 action is at the surface of the sun itself. The old theory of a 

 comparatively cool nucleus is here given up, ai d the -un is 

 allowed 10 be a glowing body, molten, doubtless ; but still the 

 most intense aciion is suppo ed to take place on the surface of 

 the sun. With regard to the spots I think the idea of Sir 

 William 1 horn-' on at that time was that there were great whirl- 

 winds at the surtace of the sun from time to lime which blew 

 away these meteors, and consequently caused, where they 

 existed, a less intense succession of impncts, and consequently 

 less hi at, . nd that a portion became comparatively dark. I 

 just mention this hi. ti rically. I will not at present say anything 

 about the very important information which the spectrt.scone 

 gives us respectirg the sun, but will reserve that to a later 

 period. 



A different theory was thrown out by M. Faye in i865' Ac- 

 cording to thi^ the interior of the sun is intensely hot, and for 

 that very reasun, as M. Faje supposed, cmparatively speaKirg 

 non-lummous. He conceived, in fact, that the interior was so 

 hot hat bodies were there in a state of dissociation ; and as we 

 know that mai.y a glowing gas gives out plenty of heat, but 

 com| aratively In tie light, so it was supposed that the interior of 

 the s\in, by virtue of its inten; e heat, radiated only comparatively 

 little light, and that it was not until the substances of which the 

 sun was cc.mpjsed can e to the outside that they became cf ol 

 en- ugh to enter into chtmical combinations, and to supply us 

 with substances which were capable of emitting an abundance 

 of light. Now here there is one feature in common with the 

 old \iews, namely, that the source of the light is supposed to be 

 a photosphere surrounding a solid body which is, comparatively 

 speaking, dark ; but the reason why this body is supposed to be 

 dark is precisely the reverse of that which was supposed in the 

 older views. In the nlder views the body of the sun was sup- 

 posed 10 be comparatively cool: here it is supposed to be so 

 intensely hot that ihe suhslances of which it is com| osed have 

 not yet s^ot into a state in which they can emit much li,i;ht. 

 According to this theory the spots are places where the photo- 

 sphere is, so to speak, blown away, and you see down into the 

 intensely hot body of the sun, which is com 1 aratively feebly 

 emissive of light. 'I his view seemed to receive some support 

 from a remarkable discovery made by Mr. Hu^jgins in 1S58 with 

 reference to the constitution of the planetary nebulae. On 

 appljing ihe 'pectroscoi e to these planetary nebulas he made a 

 remarkal le di covery, that the spectrum w hich they emit censists 

 exclu ively of bright line-, such as the spectrum we know to be 

 produced by an incandescent gas. Many of these nehuloe have 

 a somewhat stellar nucleus, v^hich seems to exhibit a spectrum of 

 a more oi'dinary character. Now at first sight this condition of 

 things appeared to be ju t what the theory of M. Fay required, 

 and to give an explanation of the phenomena according to that 

 view. The. e \ lanetary nebula: give 1 ut a feeble light compared 

 w ith the stars ; and so, when seen through an aperture in the 

 photosphere, we may suppose that the inferior gaseous portions 

 of the sun are too hot to glow wiih more than this feeble light. 



Now that suppo.' ition is in contradiction to a very important 

 extension of Prevost's theory of exchanges which was made in- 

 dependently by Prof. Balfour Stewart — who is here present, I 

 am happy to see — and by Prof. Ku-chhoflf. According to Prevos', 

 if you have a body contained within a heated envelope, and 

 everything has come to its final state, and this envelope is 



op.aque, then all the bodies within it w ill be of the same tem- 

 perature. They will receive as much heat from the walls of the 

 eiivclope as tbey give out by radiation, and there will be a 

 perfect balance between the radiation and the absorption. If 

 one of tbo:e bodies is comparatively transparent, letting thrcugli 

 a good part of the heat wi ich it receives from the envelope, it 

 will give out itself comparatively little heat, otherwise it would 

 gradually becoine cooler. Now the extension I have mentioned 

 is that this is true not merely of the sum total of the heat given 

 out or absorbed, but of each p.articular kir.d of heat or liglit of 

 which that total consists ; so that if we take light or heat of any 

 degree of refrai gil ility, there is a balance between what is 

 absorbed and what is given i ut. 



Now this exteirsion of Prevost's theory militates against M. 

 Faye's theory of the constitution of the sun as regards the con- 

 stitution of the spot-. For, take the interior of the sun. If 

 we take light cf any particular degree of refrangibility, the 

 brdy, that is, this supposed gas which constitutes the bulk of 

 the sun, will be either opaque as regards that kind of light, 

 or trsnsiiarent, or partially tiansparent. If it is opaque it is 

 certain to emit light of the same refrangibility. If it is trans- 

 parent, then the spot would not be dark, because, as rega-<!sany 

 kind of light for which this interior gas was wholly transparent, 

 we onght to see the opposite side of the photosphere shining 

 through ; just as in the planetary nebulse we do see what we 

 have every reason to suppose to be a imcleus of the nebula shining 

 right thi-ough its enormc us semi-diameter. The .tars ■ ubttnd no 

 appreci'ble angle, but the planetary nebulae subtend a very 

 appreciable angle, which can be u easured, and in all \ robability, 

 judging by tbe distarce of the planetary nebula; from us, their 

 diu ensio:is are gigantic as compared with the average size of the 

 stars, and as compared in all probability with our own .sun. 

 Therefore there ought to be seen in the sun, on that 'upposilion, 

 the same phenomenm as is seen in these planetary nel ulse, 

 nam-ly, the photosphere on the far side shining acro-s the 

 gaseous globe. It seems 10 me that that consideration is fatal to 

 the acceptance ( f M. Faye's theory as a whole, and that we must 

 have rec'iurse to some other. 



Now I have mentioned already Sir William Thomson's 

 meteoric theory, in which is involved the very important consi- 

 deration of the conver ion of work into heat. I do not mean at 

 ail, in stating some possible objectims to that theoij (which he 

 has him; elf since given up), to go against the supposition that the 

 original source of the sun's heat may have been the conver ion of 

 work into heat, but starling with the sun as it was some ages 

 ago, has the ^ubFequent heat been derived from itself, or from 

 the outside? According to tbe theory of M. F.aye, tbe heat 

 would t e derived from the sun itself, which would be spending 

 its beat gradually. So far (giving my own view as to w hat is 

 probable) it ^eems to me that the probabilities are in favour of 

 that part of the theory. Well then, if the spots are not due to the 

 dark body of the sun being exposed by something being removed 

 from tbe outside, fe it that the body is r'ark from a deficiency 

 of heat or from an excess of lieat, what may we suppose them to 

 be ? In a paper i.ul ilished in the Phi'osophual Transaction! by 

 Messrs. De la Rue, Stewait, and Lowy, the authors have 

 advocated the view that the spots are due, not to an upiising 

 from the centre of the sun, but to a dovvn-p ur of cooler prrtions 

 of the matter which has Veen ejected from the sun. But here I. 

 think I canniit go on without going back to some researches in 

 which the sptctroscope plays a most impoitant part. It is to 

 Prof. Kirchhoff ib.at we owe the first extensive application of the 

 spectroscope to the study of the sun. He held that since bodies 

 in the state of incandescent gas give out bright lines in their 

 spectrum, according to the extension which he made, indepen- 

 dently of Prof. Balfour Stewart, i f Prevost's theory of ex- 

 changes, these glowing gases ought to absorb light of the same 

 refrangibility coming frcm a body behind. Now if you had a) 

 glowing sas in front of an opaque body glowing at the same 

 tempt rature, you ought to see neither dark nor bright lines, for 

 ihe gas wruld absorb the hght of the refrangibilities which itself 

 gives out, and it would m.t absorb the light of the refrangibilities 

 which it does not give rut, so that in the region of the bright 

 lines we should, even if the body behind were away, get the fnll 

 amount of light due to the temperature, coming from tlie glovving 

 gas itself ; in other regions where there is no.uch bright light 

 coming from the gas, ycu get the full amonnt of light coming 

 from the opaque b' dy behind. But if you luppo e this gas in 

 front, glowing though it be, to be at a lower temperature than 

 ihe opaque body behind, then it would absorb more light of the 



