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SCIENCE. 



[Vol. XVII. No 430 



tributed into boundless areas of space. All this is supplied 

 by the contraction of the sua's bulk. It is in place now to 

 specify some of the interacting and counteracting forces 

 involved in this process of shrinkage of diameter and radia- 

 tion of heat. We shall more clearly see causes of inevitable 

 disturbance of equilibrium in the constantly varying energy 

 of the different factors which play unequally against each 

 other. 



Every atom of the solar orb is being continually drawn 

 towards its centre by the gravitation of the sun's own tremen- 

 dous mass ; but this tendency is resisted by the intense heat, 

 which causes each particle to repel its neighbor, and so to 

 prevent and delay that condensation to compact solidity 

 -which is to be the ultimate result. Heat must be parted with 

 before the strenuous behest of gravity can be obeyed. Thus 

 the process of contraction goes on with extreme slowness, 

 only by means of, so to speak, the squeezing-out of immense 

 volumes of heat from the whole mass. The result is an 

 imperceptible contraction of bulk, leaving the sensible heat 

 practically undiminished, although latent heat has been 

 copiously expended. The heat thus continually released, and 

 oozing from every molecule throughout the bulk of the mighty 

 orb, finds escape from the interior to the surface by means of 

 vast upboiling currents of superheated fluid which carry out 

 the heat; in other words, by the process of convection. 



Now observe the elements of variation as found in the 

 interacting forces involved. The primary factor in this 

 combination is the force of gravitation ; but gravitation must 

 increase inversely as the square of the sun's radius. As the 

 bulk shrinks, gravity multiplies. When the sun had twice 

 its present diameter, its particles drew together only one- 

 fourth as hard as they do now. Here, then, is a steadily 

 changing factor tending to disturb the uniformity of the heat 

 emitted. 



A second ever-changing factor is the area of the radiating 

 surface of the photosphere. This varies, not inversely like 

 gravitation, but directly as the square of the sun's radius. 

 When the sun was twice its present diameter, the area of its 

 photosphere was four times as extensive: in other words, the 

 heat had four times as wide a gate to find escape through. 

 This, again, tends to disturb uniformity in the emission of 

 heat. 



A third element of variation is to be found in molecular 

 repulsion, which varies not only with the amount of sensible 

 heat, which is possibly still rising as the sun grows denser, 

 but it will also vary as the square of the decreasing distances 

 between the crowding molecules. This influence is opposed 

 to that of gravitation, and tends to prevent condensation. 

 This varying quantity constitutes a third antagonist in the 

 fray, as the war sways to and fro in the sun's interior. 



A fourth factor is the slowly lessening distance from the 

 sun's centre to its surface, which facilitates the transit of the 

 outgoing currents conveying to the surface the superheated 

 fluids of the contracting interior. As the sun shrinks, the 

 path to the surface shortens directly as the radius, thus tend- 

 ing to increase the escape of heat. 



But counteracting this is the increasing density of the sun's 

 contents, which varies inversely as the cube of the radius ; 

 that is, as the shrinkage of bulk. The mass of the sun is 

 now eight times as dense as when of twice its present diame- 

 ter. This greatly increases the resistance to movement of 

 internal currents, just as one hundred people in a hall of a 

 given size will move about more than twice as easily as two 

 hundred people in the same hall who crowd and jostle each 

 other. 



A sixth and perhaps very variable factor which power- 

 fully retards the radiation of light and heat, is the enveloping 

 atmosphere of the sun, estimated at several thousand miles 

 in depth, and of considerable density. This atmosphere, 

 like an enswathing blanket, arrests a large portion of the 

 radiated heat. Now, the quantity of this atmosphere being 

 assumed as constant, its depth will tend to vary inversely as 

 the area, that is, as the square of the sun's diameter, and so 

 the radiation of heat be hindered increasingly as the sun 

 shrinks. 



It is quite impossible, however, that the quantity of atmos- 

 phere outside of the photosphere should remain exactly con- 

 stant. Large quantities are evidently carried down into the 

 sun's interior by the plunging rush of the sun-spot vortices, 

 no doubt to boil up again to the surface. 



Added to the regular atmosphere are the red cumulus pro- 

 tuberances above the atmosphere, composed of more tenuous 

 vapor forced out perhaps by electric repulsion. These must 

 contribute to arrest the escape of heat, and are also variable 

 in quantity. 



This brings us to another probable element of a perturbing 

 nature in its influence upon the escape of heat ; that is, elec- 

 trical repulsion. It is probably this which not only drives 

 forth the red protuberances to such an enormous height, but 

 which also shoots out the broad streamers of the sun's corona. 

 The tails of comets are probably forced outwards by a similar 

 repulsion from the sun. 



As this force is habitually attendant upon molecular ac- 

 tivity and the generation of heat, it must be subject to 

 considerable fluctuation with the violent internal agitation 

 of the orb. To all this fluctuation the earth's magnetism 

 constantly responds, like a delicate galvanometer. How 

 much more powerfully, then, must the sun's own atmosphere 

 respond, dilating and bristling out with every rising wave of 

 electrical agitation ! Such d ilatation of the atmosphere and its 

 vast appendages cannot fail to diminish the radiation of heat, 

 like a bird roughing its feathers in the cold. 



Miss Agnes Gierke describes those stars in the same class 

 as our sun as being more strongly electrified than the others, 

 and hence likely to be more active in their fluctuations of 

 repelling force. 



Recent developments in chemical science promote belief in 

 the existence of elementary forms of matter not yet actually 

 observed. Certain peculiarities in the spectrum of the sun 

 are thought to indicate that much of its matter is still in 

 such elementary forms, owing to its intense heat. This in- 

 creases the probability that great chemical processes are going 

 on in the sun, which are attended with evolution of heat, 

 and which thus contribute to the complexity of causes pro- 

 ducing variation thereof. 



Should we adopt the conjecture of Mr. Proctor and others, 

 that the supply of heat in the sun is largely maintained by a 

 bombardment of meteorites supposed to be densely swarm- 

 ing about it, we might find in this another element of varia- 

 tion. This is, however, hardly more than unsupported con- 

 jecture. 



The foregoing enumeration of certain and probable factors 

 in the sun's internal activity, as contributing to produce much 

 variation in the resultant emission of heat and light, is ne- 

 cessarily but rude and imperfect; yet at least it serves to illus- 

 trate and lend probability to the hypothesis advocated in this 

 essay. Some of the causes of fluctuation named seem most 

 adapted to produce comparatively brief and transient in- 

 equalities of radiation, such as might easily be verified by 

 long-extended instrumental measurements in elevated posi- 



