120 



KNOWLEDGE & SCIENTIFIC NEWS. 



[June, 1904. 



that the familiar eleven-year period is resolvable into the 

 separate progresses of three or four simpler elements, 

 which overlap one another and severally arise and dis- 

 appear within seven or eight years; and a consideration 

 of the Stonyhurst magnetic records by Father Cortie has 

 served to prove how indefinable in the present state of 

 our knowledge is the bond coimecting magnttic storm 

 with solar outbursts. In the presence of rapid and 

 promising developments on the one hand, and increasing 

 doubt upon the other, reserve in formulating any theory is 

 unavoidable. No great confidence can be placed in 

 arguments not based upon consideratijns which are of 

 the widest generality, and cannot under any circum- 

 stances lie falsified; such, for example, as the laws 

 by which such a body must graduilly condense under the 

 infiuence of gravitation and loss of heat. Even here it is 

 necessary to make somewhat sweeping assumptions before 

 any precise conclusions can be drawn, and there is con- 

 siderable disagreement among the results at which 

 ditferent authors have arri\ed. Vet 1 believe some plain 

 and necessary outline can be drawn which will co\er 

 many of the most prominent facts. Though such an in- 

 vestigation relates more directly to the conditions prevail- 

 ing within the body of the Sun than to the state at the sur- 

 face, with which it might at first appear that we were alone 

 concerned, its bearing upon the latter question is intimate. 

 Thus Professor Schuster has said that the main differ- 

 ence between stars which show a spectrum like our Sun, 

 filled with metallic absorption lines, and those which, like 

 Vega, show only the absorption of hydrogen, is neither 

 m(jre nor less than the UKjre thorough mixing up of the 

 atmospheres of the former ; " if we could introduce a 

 stirrer into a Lyrac there can be no doubt whatever 

 that the low-temperature lines of iron would make their 

 appearance." If this be true, the stirrer we are seeking 

 consists of more or less violent convection currents, and 

 in order to form a just estimate of how efficient these 

 may be we should study that instability which in great 

 or small degree is always present where convection 

 currents exist. 



Instability with bodily interchange of mateiial does 

 \isibly exist in the Sun, and must do so, or else its face 

 would soon be covered with a dense luask of relatively 

 cold matter; it is radiation which sets this instability up, 

 and the key to understanding it is some comprehension 

 of the process of radiation. For example, it should be 

 realized very clearly that a comparison of the radiant 

 energy emitted by two bodies is no comparison of their 

 temperatures unless they are in similar stales ; thus a 

 solid body maintained at a certain temperature radiates 

 sensibly as from its surface; but if it be finely divided, 

 and its parts scattered, it will radiate enormously faster 

 from the same temperature, since its surface will be 

 enormously nuiltiplied. Mence, if a sunspot appears 

 nearly black in comparison with the rest of the disc, or 

 if, as in M. Janssen's photographs, we see the whole 

 surface mottled over with minute brilliant spots upon a 

 darker background, the simplest explanation is that the 

 brighter parts represent matter diffused in cloud, and the 

 darker parts are relatively dense and conglomerate. 



In my opinion, the whole internal state is dominated 

 by radiation, for apart from this source of loss of heat, 

 there is no reason why the body should not settle down 

 to any law of distribution of its matter in which the 

 density did not increase from the centre outwards. But 

 I must profess myself a total disbeliever in the state 

 of affairs whicli it is commonly asserted would in con- 

 se<]uence arise. 



This state is Lord Kelvin's well-known " (^onvective 

 Equilibiinm " of temperature. I )iscuisiiig in 1862 the 



state of the earth's atmosphere, and observing how winds 

 and other currents mingled together with great rapidity 

 portions of air which had been widely separated. Lord 

 Kelvin adopted the hypothesis that the temperature at 

 different levels must be such that this indifferent mingling 

 should not change it ; in other words, the excess of heat- 

 energy possessed by a portion of air at a lower level of 

 the atmosphere, and at consequently greater pressure and 

 density, must be just sufficient to expand the same por- 

 tion to a pressure and density in equilibrium with those 

 at any level above to which it may be transported. The 

 same law he afterwards adopted as regulating the whole 

 internal state of the Sun, and many other eminent 

 authorities have followed him, the latest and not the 

 least of whom is Professor Schuster. If it is true of the 

 Sun, we must allow that the Sun's density diminishes 

 somewhat rapidly from the centre outwards, while the 

 temperature from the surface to the centre rises with a 

 great rapidity, which is maintained without much decline 

 right throughout the whole body and reaches millions of 

 degrees centigrade before one-tenth of the radius has 

 been measured. 



No doubt we must be prepared for some extravagances 

 in theorising upon matters so little known, but it is 

 at any rate safe to keep as far as possible from tempera- 

 tures measured in millions of degrees ; and in spite of 

 the long acceptance of the theory of convective equili- 

 brium in the Sun and the formidable array of authority 

 by which it has been adopted, I confess I can find no 

 reason why it should be supposed to exist. On the 

 contrary it appears to me that if we can imagine it to be 

 artificially set up, it would require forces to maintain it 

 for which the circumstances make no pro\'ision. For if 

 a body of gas were arranged according to this law, behind 

 some screen which prevented it from losing energy by 

 radiation, and the screen were then removed, wliat would 

 happen ? All portions would commence to lose heat, the 

 outer portions very rapidly by mere radiation; but the 

 inner portions also, in part by radiation, because they 

 were less screened outwards than inwards, but chielly 

 because the outer chilled portions which had already 

 lost the heat that allowed them to maintain themselves 

 at the higher level descended upon them and shared in 

 their stores. This would go on without any attempt 

 on the part of the body of gas to restore the state of con- 

 vective equilibrium, because no instability would occur 

 which would give rise to convective currents mingling to- 

 gether the matter from separated regions, until the body 

 had departed materially from the rapidly- varying density of 

 convective e(iuilibrium and had passed that of a density 

 uniform throughout the mass. Even then the currents 

 would only be proportionate to the degree by which a 

 uniform density was overstepped, and except at the outer 

 surface, where it is impossible to escape from a high 

 degree of instability and conser]uently violent convective 

 currents, the density would apparently be left in a state 

 which might perhaps fluctuate a little, but would be but 

 very little removed from a state of uniformity. It would 

 follow that the temperature was also substantially the 

 same throughout the bulk. Or again, if we reverse our 

 attitude and suppose a body set up with density and 

 temperature nearly uniform throughout its body, but on 

 the whole very slightly increasing outwards and therefore 

 liable to slight convective currents— excepting at the 

 surface — and ask what forces would be found which could 

 materially disturb such a state, none can be mentioned. 

 Kadiation which appeared as an acting cause lending to 

 set up such a state will be inoperative when that state is 

 attained — excepting again the surface — and conduction 

 also, if we cho'e to consider it, would be inoperative with 



