342 



NATURE 



[August II, 1887 



regarding them in the three concluding sections of his 

 work on Cosmogony, he by no means underrates the 

 difficulties thev present. The range of our sensible 

 experience shrinks into absolute insignificance when com- 

 pared with the exalted conditions reigning in the sun. 

 The temperature at its surface may well reach 40,000° to 

 100,000° C. ; near the centre it mounts probably (our 

 author considers) to ten, possibly to thirty or more million 

 degrees. This unimaginable vehemence of heat is 

 bafanced by an unimaginable urgency of pressure. The 

 statement that, in the depths of the sun's interior, it reaches 

 a maximum of 2,000,000,000 atmospheres gives only 

 nominal expression to its value. Figures can at times 

 keep pace with facts only on the condition of being 

 reduced to empty and meaningless symbols. 



Gravity and molecular motion— the two universal ant- 

 agonists—here carry on a conflict intensified far beyond 

 the control of " laws " derived from terrestrial observation. 

 The correlation between elasticity on the one side, and 

 pressure and temperature severally on the other, estab- 

 lished by Boyle and Gay-Lussac, holds good only over a 

 strictly limited range of conditions. Calculations founded 

 on the supposition of its continued prevalence in the sun 

 lead at once to manifest incongruities. Solar speculators 

 are thus left, to a great extent, without the guidance of 

 ascertained principles. In the region frequented by them, 

 the scientific imagination has free play. Apposite facts 

 are scarce ; misleading analogies too much abound. It 

 cannot then be wondered at if theories of the sun often 

 include extravagances which it is easier for their critics to 

 discern than for their constructors to avoid. 



A futile debate has sometimes been raised as to 

 whether the interior constitution of the sun is liquid or 

 gaseous. The truth seems to be that neither word is 

 properly apphcable. Without unduly stretching its ori- 

 ginal meaning, neither describes with even approximate 

 accuracy the state of things prevailing there. The notion 

 of " critical points " has been called in question, and may 

 be inexact. But its introduction has at least had the not 

 unimportant effect of abolishing an artificial distinction. 

 It has shown the separation of the various st:ites of matter 

 to be merely provisional. Their characteristic qualities 

 depend upon circumstmces for their development or 

 maintenance. At transcendental temperatures and pres- 

 sures, the ordinary— probably (as Dr. Braun remarks) 

 even the scientific— criteria of gases and liquids disappear ; 

 one state merges into the other ; they interchange natures ; 

 so that we may indifferently regard the sun's interior as 

 composed of vapours compressed, in despite of their 

 almost boundless calorific energy, to the consistence of 

 fresh putty, or of liquids restrained from boiling by the 

 main force of the strata loaded upon them, while expanded 

 to four or five times their ordinary bulk, and rendered 

 internally mobile by the prodigious elevation of the 

 temperature. An indisputable fact, however, and one 

 fundamental to solai- physical theory, is that the sun 

 constitutes a vast reservoir of opposing, tremendously- 

 constrained forces, the delicate equilibrium of which 

 cannot be disturbed, however slightly, without producing 

 effects on a commensurate scale. 



Upon such inevitable disturbances Dr. Braun founds 

 his ratiofiale of the more obvious solar phenomer. i. The 

 cooling of a body like the sun does not assuredly proceed 

 quite equably. Local excesses of temperature lead to what 

 we may call local revolts against gravity, signified by swift 

 uprushes from great depths of inconceivably heated sub- 

 stances. These are the so-called " metallic prominences." 

 But where the forces called into play lack energy to pro- 

 duce, or the attendant circumstances are not sufficiently 

 favourable to permit, an actual outbreak, an uplifting of the 

 unbroken photospheric surface takes place, and we see a 

 " facula." "Hydrogen-prominences" mark a medium 

 stage of vehemence. They originate from a commotion 



which primarily fails to outpass the limits of the chromo- 

 sphere. The injection, however, into it of a prodigious 

 bulk of metallic vapours rapidly heats the circuiT;ijacent 

 hydrogen ; it spouts upward in a stream which aerostatic 

 pressure tends to perpetuate, and forms, high up above 

 the sierra-edge of the agitated ocean it springs from, a 

 rosy cloud conspicuous by reason of its incandescence. 



But the connexion here indicated, to be significant, 

 should be invariable, which is very far from being the 

 case. Metallic intrusions into the chromosphere are by 

 no means a condition sine qua non to the development of 

 quiescent prominences. 



Solar theorists are now for the most part agreed that 

 spots must be ascribed immediately to falls of relatively cool 

 matter upon the photosphere ; they divide on the question 

 whethei the initial disturbance comes from beneath or 

 above it. Dr. Braun ranges himself on the side of those 

 who assimilate outbursts on the sun to volcanic commo- 

 tions on the earth. Uprushes of vividly glowing sub- 

 stances due to the temporary preponderance of heat over 

 pressure are answered by downrushes of obscure absorbing 

 vapours. Spots would thus be the reactive effects of 

 flames or prominences. Their occurrence would be im- 

 possible without preliminary eruptions. But it is at least 

 doubtful whether in this hypothesis the real sequence of 

 events be not inverted. The whole tenor of Mr. 

 Lockyer's observations goes to prove that the yawning of 

 the photosphere leads the way as a symptom of its 

 agitation. After a spot has begun to form, its flame and 

 facular garni things are added. M. Trouvelot has, indeed, 

 often perceived a nascent spot to be completely masked 

 by towering masses of faculai ; but it is none the less 

 there, waiting to be disclosed. Prof. Young considers the 

 appearance of a spot to be commonly heralded by mani- 

 fest disturbances of the surface ; but since the disturbance 

 is evidenced as well by the presence of "pores" (which 

 may be termed embryo spots) as of faculse, his authority 

 can scarcely be invoked as decisive of the question of 

 precedence. 



This is really the touchstone of the rival theories. 

 Outbursts from the photosphere are either the cause or 

 the consequence of the obscurations of it termed " spots." 

 If the former, they should unfailingly and unmistakably 

 take the initiative. But facts certainly warrant no such 

 rigid conclusion. Admitting then the alternative order of 

 connexion, we can understand that descents of relatively 

 cool matter from coronal regions, perforating the photo- 

 sphere, must overturn the precarious equilibrium of heat 

 and gravity reigning beneath it, and may thus occasion 

 the tumultuous heavings visible as facute, and the 

 amazing escapes of imprisoned vapours challenging atten- 

 tion as flames. 



Dr. Braun's papers on the constitution of the sun were 

 published in Natur und Offenbarung previous to the 

 appearance of Mr. Lockyer's " Chemistry of the Sun." 

 Hence, perhaps, his complaint that the observed facts 

 regarding the solar rotation had as yet been in:luded in 

 no " plausible " hypothesis. We cannot think him suc- 

 cessful in his effort to supply the want. 



Adventitious arrivals of nebular supplies from inter- 

 stellar space play, as our readers are already aware, an 

 indispensable part in the theory of planetary development 

 sketched in the earlier chapters of the work now, in its 

 concluding portion, engaging our attention. By their 

 agency the primitive nebula was set whirling with a 

 motion accelerated outward, its central sluggishness per- 

 sisting throughout, and modifying the whole of its long 

 history. The inequality is perpetuated within the body 

 of the sun itself, the innermost parts of which may require, 

 our author thinks, as much as forty or fifty days to com- 

 plete a rotation performed at the equatorial surface in 

 twenty-five. The quickening of angular rate continues 

 v/ith ascent into the solar atmosphere, until, in its higher 



