246 



NA TURE 



\yuly 30, 1874 



sented to the mind is that the head of a comet is the seat 

 of an emission of matter which takes place in a direction 

 opposite to the sun ; it seems as if the comet fused at 

 one end, and that the matter thus thrown off is arranged 

 into an immense phime, exactly like the smoke which 

 escapes from the chimney of a steamer at full speed. 

 Let us examine this analogy more closely, and suppose, 

 first, the boat to be motionless, with the smoke ascending 

 vertically in a perfectly calm atmosphere. Each puff of 

 this smoke is sent into the air with a certain speed, and 

 the successive sections of the vertical plume thus formed 

 will represent the positions which these puffs will have 

 reached at the same instant. The puffs first emitted will 

 be the highest ; the latest ones will be lowest ; if then we 

 knew the law of the ascending movement of any puff, we 

 should thus be able to assign the instant at which each 

 section of the vertical plume was shot forth. Meantime, 

 should we set the steamer moving in the motionless air, 

 the place at which each section is emitted will gradually 

 advance ; each of these will ascend almost vertically over 

 its place, for the speed of the horizontal movement which 

 the boat communicates to it will be very rapidly exhausted 

 in resisting the motionless air, and at the end of a certain 

 time these pufl's will be found dispersed in an inclined 

 plume, presenting a curvature more or less marked. At 

 first, this curvature will assume a vertical direction, i.e. 

 the direction of emission. 



On the other hand, the successive puffs, in ascending, 

 tend to spread out ; the earliest and highest must then 

 become rarefied and disappear from sight. The tail, — 

 no, 1 should say the plume of smoke thus formed, must 

 become less and less dense, at the same time becoming 

 less and less distinct and gradually getting obliterated. 



Does it not seem as if here we had put our finger upon 

 a complete analogy ? The comet proceeds on its way like 

 a steamer ; it describes round the sun an orbit elongated 

 like the path of a bomb ; heated more and more by the solar 

 rays, its matter is expanded and escapes into space, like 

 that of a rocket. Is it not natural that it should send off 

 a plume analogous to that which escape; from the funnel 

 of a machine in motion ? If we knew the rate of emission 

 of each pul'f of cometary vapour, would we not be able to 

 calculate the place which it must occupy in the tail, 

 and even the form of the tail itself? Reciprocally, after 

 having carefuliy determined the figure of this tail, would 

 we not be able to form some estimate of the rate of the 

 nucleal emission of the comet .■" Such, very nearly, was 

 Newton's point of view in studying these magnificent 

 phenomena. The comet of 16S0, which appeared in the 

 time of Newton, had a tail of 25,000,000 leagues in length ; 

 it forcibly impressed this great geometer, and originated 

 in his mmd views similar to the analogy which we have 

 just indicated. 



But analogy is not always a perfectly trustworthy 

 guide. Here the differences preponderate consider- 

 ably over the likenesses. We have certainly in the 

 heavens a heated body which in its progress emits 

 vapours like a gigantic steamer ; but where is the 

 funnel, where is the atmosphere ? And, remember, 

 the atmosphere here plays an important part, for it 

 is its presence which determines the ascent of the 

 puffs of smoke. If these ascend, it is from the same 

 cause as balloons, because they are lighter than air. 

 Take away the air, instead of mounting they will fall. 

 Well, in the sky there is no air ; space is void of matter 

 forming a continuous and ponderous medium, layer on 

 layer, until the surface of the sun is reached. Moreover, 

 Laplace has shown ihac the power of the Siin in attrav^t- 

 ing a ponderable fiuij will not extend beyond a veiy 

 narrow limit. As to the ether of the physicist, it need not 

 engage our attention for an instant, since, by definition, 

 this hypothetical ether is imponderable. We shall not be 

 much astonished that the genius of Newton should have 

 been content with a similar analogy, if we only reflect on 



all the difficulties which the doctrine of attraction raised 

 in the minds of the eighteenth century, and on the Carte- 

 sian prejudices which greeted its first appearance on the 

 Continent. What would have happened if, at the first, 

 the too absolute terms of this doctrine had seemed 

 to be contradicted by the phenomena of the figure of 

 comets ? It was then necessary, at any cost, after having 

 incontestably connected the movement of these bodies 

 with the new doctrine, to let it also be seen, even though 

 it was by an analogy somewhat forced, that their figure 

 could be explained in the same manner. 



Now that the doctrine of attraction is established on 

 an immovable foundation, our mind is able to detach itself 

 from the purely metaphysical part of the original affirma- 

 tions, which presented it to us as the single force to which 

 all celestial phenomena ought to be subordinated. But 

 before invoking another force, it is necessary at the very 

 outset to draw from attraction all the consequences ap- 

 plicable to comets ; and we shall do so by showing that 

 the force, which seems constantly to tend to unite, to 

 agglomerate scattered material, is, in reality, also quite 

 capable of producing in certain cases the opposite effect, 

 viz., of undoing existing agglomerations. 



To proceed in order, let us ask, first, why comets have 

 tails while planets have not. Is it because comets ap- 

 proach closer to the sun and are thus subjected to a very 

 powerful heat .'' Certainly not ; for the planets \'enus an J 

 Mercury, especially, are constantly closer to the sun than 

 most of the coni'ls at their perihelion, and yet neither 

 Venus nor Mercury has the faintest trace of a tail. Must 

 we attribute the figure of comets to the parabolic nature 

 of th'.ir ovb'ts, in virtue of which their distance from the 

 sun varies enormously, while the plmets remain always 

 very nearly at the same distance from the centre of our 

 solar system ? An illustrious poet, Lamartine, wishing to 

 depict a creator of the earth, indifferent to his creature, 

 has beautifully said — 



Et il'im pie I rleJaigneux la lanfint dans I'espac'c, 

 Uentra dans son repos. 



If the kick had been stronger, the e.arth would have 

 been sent to describe a cometary orbit round the sun, i.e. 

 an elongated ellipse or a parabola, instead of the circle 

 which it now describes ; but, for all that, it would not 

 have become a comet, it would have had no tail. Do 

 you know what shape would be the result on this suppo- 

 sition .-' The imperceptible solar tides of the ocean would 

 be gradually restrained in proportion as the earth in ■ 

 creased its distance from the sun, and soon would dis- 

 appear altogether ; nur atmosphere would be more and 

 more condensed into layers always spherical and concentric 

 with the earth ; our planet would be lost in the depths of 

 infinite space without any other change than a more 

 marked contraction due to the predominating cold of 

 space. 



Are comets, then, formed of matter dit'ferent from that 

 of the planets 1 No ; such an idea cannot be accepted 

 now that spectrum analysis has told us of the existence 

 of sodium, magnesium, and calcium in the sun, hydrogen 

 in the stars, and our ordinary gases even in the most 

 distant nebuUe. Above all, we find the same elements 

 subject to the same mechanical, physical, and chemical 

 laws. 



The truth is more simple. If our planets have no tails, 

 it is becau-e they have an enormous mass ; if comets 

 have tails, it is because their mass is extremely small, and 

 because the attraciion which this mass exercises upon 

 their materials is not sufficient to hold them back and to 

 overcome the external forces which tend to decompose 

 them. 



Nowhav-wehit upon a notion which I must dwell 

 upon all the more that it has not hitherto been sufficiently 

 popularised. You have heard of a general law in tl e 

 world of organised and living beings, called '' the struggle 

 for life," the fight or effort which it is necessary to make 



