Sept. 1 8, 1884] 



NA TURE 



505 



enhanced by the enormous difference between solar conditions 

 and the conditions attainable in our laboratories. We often 

 reach, indeed, similarity sufficient to establish a bond of connec- 

 tion, and to afford a basis for speculation ; but the dissimilarity 

 remains so great as to render quantitative calculations unsafe, 

 and make positive conclusions more or less insecure. We can 

 pretty confidently infer the presence of iron and hydrogen and 

 other elements in the sun by appearances which we can re- 

 produce upon the earth ; but we cannot safely apply empirical 

 formula; (like that of Dulong and Petit, for instance), deduced 

 from terrestrial experiments, to determine solar temperatures : 

 such a proceeding is an unsound and unwarrantable extrapolation, 

 likely to lead to widely erroneous conclusions. 



For my own part, 1 feel satisfied as to the substantial correct- 

 ness of the generally received theory of the sun's constitution, 

 which regards this body as a great ball of intensely heated 

 vapours and gases, clothed outwardly with a coat of dazzling 

 clouds formed by the condensation of the less volatile substances 

 into drops and crystals like rain and snow. Yet it must be 

 acknowledged that this hypothesis is called in question by high 

 authorities, who maintain, with Kirchoff and Zbllner, that the 

 visible photosphere is no mere layer of clouds, but either a solid 

 crust, or a liquid ocean of molten metals ; and there may be 

 some who continue to hold the view of the elder Herschel (still 

 quoted as authoritative in numerous school-books), that the 

 central core of the sun is a solid and even habitable globe, 

 having the outer surface of its atmosphere covered with a sheet 

 of flame maintained by some action of the matter diffused in the 

 space through which the system is rushing. We must admit 

 that the question of the sun's constitution is not yet beyond 

 debate. 



And not only the constitution of the sun itself, but the nature 

 and condition of the matter composing it, is open to question. 

 Have we to do with iron and sodium and hydrogen as we know 

 them on the earth, or are the solar substances in some different 

 and more elemental state ? 



However confident many of us may be as to the general 

 theory of the constitution of the sun, very few, I imagine, would 

 maintain that the full explanation of sun-spots and their be- 

 haviour has yet been reached. We meet continually with 

 phenomena, which, if not really contradictory to prevalent 

 ideas, at least do not find in them an easy explanation. 



So far as mere visual appearances are concerned, I think it 

 must be conceded that the most natural conception is that of a 

 dark chip or scale thrown up from beneath, like scum in a 

 cauldron, and floating, partly submerged, in the blazing flames 

 of the photosphere which overhang its edges, and bridge across 

 it, and cover it with filmy veils, until at last it settles down again 

 and disappears. It hardly looks like a mere hollow filled with 

 cooler vapour, nor is its appearance that of a cyclone seen from 

 above. But then, on the other hand, its spectrum under high 

 dispersion is very peculiar, not at all that of a solid, heated slag, 

 but it is made up of countless fine dark lines, packed almost in 

 coutact, showing, however, here and there, a bright line, or at 

 least an interspace where the rank is broken by an interval wider 

 than that which elsewhere separates the elementary lines, — a 

 spectrum which, so far as I know, has not yet found an analogue 

 in any laboratory experiment. It seems, however, to belong to 

 the type of absorption spectra, and to indicate, as the accepted 

 theory requires, that the spot is dark in consequence of loss of 

 light, and not from any original defect of luminosity. Here, 

 certainly, are problems that require solution. 



The problem of the sun's peculiar rotation and equatorial 

 acceleration appears to me a most important one, and still 

 unsolved. Probably its solution depends in some way upon a 

 correct understanding of the exchanges of matter going on 

 between the interior and the surface of the fluid, cooling globe. 

 It is a significant fact (already alluded to) that a similar relation 

 appears to hold upon the disk of Jupiter, the bright spots near 

 the equator of the planet completing their rotation about five 

 minutes more quickly than the great red spot which was forty 

 degrees from the equator. It is hardly necessary to say that an 

 astronomer, watching our terrestrial clouds from some external 

 station (on the moon, for instance), would observe just the 

 reverse. Equatorial clouds would complete their revolution 

 more slowly than those in our own latitude. Our storms travel 

 towards the east, while the volcanic dust from Krakatoa moved 

 swiftly west. We may at least conjecture that the difference 

 between different planets somehow turns upon the question 

 whether the body whose atmospheric currents we observe is 

 receiving more hea! from without than it is throwing off itself. 



Whatever may be the true explanation of this peculiarity in the 

 motion of sun spots, it will, when reached, probably carry with 

 it the solution of many other mysteries, and will arbitrate con- 

 clusively between rival hypotheses. 



The periodicity of the sun-spots suggests a number of 

 important and interesting problems, relating, on the one hand, 

 to its mysterious cause, a d, on the other, to the possible effects 

 of this periodicity upon the earth and its inhabitants. I am no 

 "sun-spottist " myself, and am more than sceptical whether the 

 terrestrial influence of sun-spots amounts to anything worth 

 speaking of, except in the direction of magnetism. Put all must 

 concede, I think, that this is by no means yet demonstrated (it 

 is not easy to prove a negative) ; and there certainly are facts 

 and presumptions enough tending the oilier way to warrant 

 more extended investigation of the subject. The investigation 

 is embarrassed by the circumstance, pointed out by Dr. Gould, 

 that the effects of sun-spot periodicity, if they exist at all (as he 

 maintains they d 1), are likely to be quite different in different 

 portions of the earth. The influence of changes in the amount 

 of the solar radiation will, he says, be first and chiefly felt in 

 alterations and deflections of the prevailing winds, thus varying 

 the distribution of heat and rain upon the surface of the earth 

 without necessarily much changing its absolute amount. In 

 some regions it may, therefore, be warmer and drier during 

 a sun-spot maximum, while in adjoining countries it is the 

 reverse. 



There can be no question that it is now one of the most 

 important and pressing problems of observational astronomy to 

 devise apparatus and methods delicate enough to enable the 

 student to follow promptly and accurately the presumable 

 changes in the daily, even the hourly, amounts of the solar 

 radiation. It might, perhai s, be possible v> ith existing instruments 

 to obtain results of extreme value from observations kept up with 

 persistence and scrupulous care for several years at the top of 

 some rainless mountain, if such can be found ; but the under- 

 taking would be a difficult and serious affair, quite beyond any 

 private means. 



Related to this subject is the problem of the connection be- 

 tween the activity of the solar surface and magnetic disturbances 

 on the earth, — a connection unque.-tionable as matter of fact, 

 but at present unexplained as matter of theory. It may have 

 something to do with the remarkable prominence of iron in the 

 list of solar materials ; or the explanation may, perhaps, be 

 found in the mechanism by means of which the radiations of 

 light and heat traverse inter-planetary space, presenting itself 

 ultimately as a corollary of the perfected electro-magnetic theory 

 of light. 



The chromosphere and prominences present several problems 

 of interest. One of the most fruitful of them relates to the 

 spectroscopic phenomena at the ba^e of the chromosphere, and 

 especially to the strange differences in the behaviour of different 

 spectrum-lines, which, according to teirestial observations, are 

 due to the same material. Of two lines (of iron, for instance) 

 side by side in the spectrum, one will glow and blaze, while the 

 other will sulk in imperturbable darkness ; one will be distorted 

 and shattered, presumably by the swift motion of the iron vapour 

 to which it is due, while the other stands stiff and straight. 



Evidently there is some deep lying cause for such differences ; 

 and as yet no satisfactory explanation appears to me to have 

 been reached, though much ingenious speculation has been 

 expended upon it. Mr. Lockyer's bold and fertile hypothesis, 

 already alluded to, that at solar and stellar temperatures our 

 elements are decomposed into others more elemental yet, seems to 

 have failed of demonstration thus far, and rather to bave lost 

 ground of late; and yet one is almost tempted to say, "It 

 ought to be true," and to add that there is more than a possibility 

 that its essential truth will be established some time i.i the 

 future. 



Probably all that can be safely said at present is, that the 

 spectrum of a metallic vapour (iron, for instance, as before) de 

 pends not only upon the chemical element concerned, but al-o 

 upon its phy-ical conditions ; so that, at different levels in the 

 solar atmosphere, the spectrum of the iron will differ greatly as 

 regards the relative conspicuousness of different lines ; and so it 

 will happen, that, whenever any mass of iron vapour is suffering 

 disturbance, those lines only which particularly characterise the 

 spectrum of iron in that special state will be distorted or reversed, 

 while all their sisters will remain serei.e. 



The problem of the solar corona is at present receiving much 



attention. The most recent investigations respecting it — those 



1 u! 1 r. Huggins and Prof. Hastings — .end in directions 



