288 



NA TURE 



\_A71g. 13, 1874 



nucleus, one of which is, to use a botanical term, of a 

 cyathiform aspect, while the other is almost conical. 



If we compare this theoretical figure with that of the 

 head of the comet of 1861 (Fig. 15) and of other comets, 

 it will account for the transparency of these surfaces and 

 for the effects of perspective. The latter are continually 

 changing, for comets are presented to us in all imaginable 

 positions.* The conical anterior envelope is often de- 

 scribed by observers under the name of a luminous sector, 

 a term which gives a false idea of its real form. In the 

 head of Donati's comet, the luminous sector appears to 

 have had an amplitude very much greater than that of 

 the comet of 1 86 1. 



You see that all the most constant, the best investigated, 

 the most characteristic details of the figure of comets 

 agree in disclosing the action of a repulsive force which is 

 exerted by the sun not in virtue of his mass, but in virtue 

 of his superficial incandescence. Extinguish the photo- 

 sphere of the sun, reduce it in thought to the state of 

 crust to which cold has long ago brought the earth, so as 

 to leave nothing more than the solar attraction inherent 

 in its mass, indestructible as the mass itself, and you will 

 suppress at the same time the gigantic tails of comets 

 and the cup-shaped emissions of their heads. They 

 will no doubt still lose some part of their materials 

 in approaching the sun, but these materials will be dis- 



seminated along the orbit of the comet instead of flying 

 away from the sun into space with an incredible swiftness. 

 In a word, comets would lose tlie forms represented in 

 Fig. 7, and would assume those of Fig. 6. 



It may perhaps appear to you singular that we must 

 go to celestial phenomena for evidence of the existence of 

 a force so widespread as repulsion due to heat when it 

 acts at a sensible distance, and not from molecule to 

 molecule. In reality there is nothing astonishing in this ; 

 it was the same with attraction. 



Each of you is firmly convinced of the existence of this 

 force ; you know that two spherical bodies attract each 

 other in proportion to their mass, and in inverse pro- 

 portion to the square of their mutual distance, and that 

 notwithstanding that you have not had ocular demon- 

 stration, that you have not tested it by experiment ; 

 around us, within us, nothing announces to us that bodies 

 attract each other. No direct experiment has ever been 

 made on the point in France, and if any physicist set 

 himself to it, he would require six months at least to 

 prepare for what is known in England as the " Cavendish 

 Experiment." 



But if attraction produces around us effects so feeble 

 that no mechanician orphysicistever thinks of taking them 



* Moreover, the least want of homogeneity in the nucleus and a rotatory 

 tnovcment may considerably modify the phenomena and leave only the 

 narrow part of the anterior caly.\. iJiit these anomalies do not Lake away 

 from the phenomenon its characteristic physiognomy.^ even when they give, 

 or example, to the anterior calyx, a most curioUs radiated aspect. 



into account in his experiments and calculations, on the 

 other hand it acts on a grand scale in celestial space on 

 account of the greatness of the masses. Well, it is the 

 same with the force of repulsion ; on account of the incan- 

 descence of the surface of the sun, of the enormous 

 extent of that surface, and of the small density which 

 matters may acquire, where they have infinite space 

 in which to expand. Although this repulsive force is 

 acting all round us, just like attraction, it is quite as diffi- 

 cult to prove it, because we cannot attain by means of 

 our furnaces the degree of incandescence of the sun, and 

 above all because we operate only upon insignificant 

 surfaces, and because we work in an atmosphere of an 

 enormous density as compared with cometary materials. 

 It is easy to see, then, that to obtain evidence it would be 

 necessary to resort to combinations as delicate as those 

 of the Cavendish Experiment. 



We may, however, do this : the repulsive force, with all 

 the characteristics which we have discovered in it, is yet 

 only a hypothesis which accounts at once for the figure 

 of comets and for the acceleration of their motion. We 

 have connected it, it is true, by the incandescence of the 



sun, with the familiar phenomena of the repulsion deter- 

 mined by heat between the molecules of bodies ; but it 

 remains to show, by a direct experiment, the difficulties of 

 which we have seen, that this repulsion exists beyond the 

 infinitely small distance which separates these molecules. 

 This experimental verification of every liypothesis is an 

 essential thing in astronomy ; by this alone can our minds 

 be fully convinced. The physicist, on the contrary, can 

 use more largely the convenient artifice of hypotheses, 

 since he holds in his hand, so to speak, the phenomena 

 which he studies, may reproduce them, call them forth at 

 his pleasure, and regard his subjects in all tlieir aspects. 

 Should an hypothesis be found to contradict certain facts, 

 the physicist imagines for them another more comprehen- 

 sive which he will subject to the same process. It is not 

 so with astronomy. That which has long been wanting 

 to the theory of attraction in the case of many minds 

 strongly prejudiced, moreover, in favour of another doc- 

 trine, is precisely this direct and experimental verification 

 the necessity for which I have pointed out. Everybody 

 did not feel, on the appearance of the Priiicipia, that it 

 was implicitly contained in the famous calculus which 

 enabled Newton to see that the force which holds the 

 moon in its orbit is identical with that which every- 



