Atig. 13, 1874] 



NA TURE 



where causes bodies to fall to the ground. The learned 

 opponents of the doctrine on the Continent would, with- 

 out doubt, have been favourably disposed to it before the 

 experiment of Cavendish or that of Maskelyne,if Newton 

 had been able to realise to them so as to show to all eyes 

 that bodies of suitable form and of any nature whatever 

 attract each other in proportion to their mass and in the 

 inverse ratio of the square of their distance. 



But how are we to apply the Cavendish balance to the 

 measurement of the repulsive force of an incandescent 

 surface ? First of all, the materials of our apparatus are 

 of a density enormously superior to that of comets ; then 

 it is necessary to operate in a perfect vacuum, fo the least 

 trace of air which remains in the apparatus will give rise 

 to currents under the influence of a surface strongly 

 heated, and will thus obscure the effect which we endea- 

 vour to establish. In trying to surmount this difficulty,* 

 I have been led to think that if I could make an incan- 

 descent surface act upon the small mass of air itself 

 which acts as an obstacle to us in the vacuum of our best 

 pneumatic machines, I should obtain a very appreciable 

 repulsion ; only we must find some means of rendering 



this air visible. The artifice to which I am about to have 

 recourse before you consists in illuminating this r.irefied 

 air by means of the spark of Ruhmkorff s induction appa- 

 ratus. (See Fig. 17.) This glass bell-jar, in which a 

 vacuum has been made, is traversed by the two conductors 

 of the apparatus, the one vertical and the other horizontal. 

 You see the spark spring out under the form of feebly 

 luminous stratifications of a peculiar rose colour ; at the 

 same time the horizontal conductor is covered with a 

 luminous sheath of a well-marked blue colour. It is the 

 air which is thus illuminated by the passage of the cur- 

 rent. Remark, however, the particular form of the hori- 

 zontal wire ; it is formed partly of a thin blade of platinum 

 surrounded by a blue aureole. 1 shall redden this plate 

 by means of an ordinary current, composed of sever.1l 

 Bunsen couples. I cause this current to pass through the 

 horizontal conductor, not disturbing in the least the first 

 induction current. The platinum plate becomes incan- 

 descent, and you soon see the blue-coloured sheath sepa- 

 rate from the platinum plate like two lips which are 

 parted. 



I have varied this e.xperiment to obviate the objections 



to which the increase of the conductibility of the air 

 might give rise ; but it has always succeeded. Thus have I 

 obtained an analogous repulsion by acting transversely 

 upon the rose-coloured stratifications ; the case was abso- 

 lutely the same as if a perfect vacuum existed around the 

 plate, a vacuum of definite limits beyond which the elec- 

 tricity would not pass, while an increase of conductivity 

 would simply cause the induction-spark to incline towards 

 the favourable region and so to modify its usual configura- 

 tion. 



Thus have we been led lo conclude with perfect cer- 

 tainty (i) that cometary phenomena reveal to us in the 

 universe the existence of a second force totally different 

 from attraction, capable of playing an important part and 

 producing before our eyes gigantic phenomena ; (2) with 

 great probability, that this force is nothing else than the 

 repulsion due to heat. 



Perhaps we may come upon this force when we investi- 

 gate more closely the strange phenomena of the solar 

 protuberances which the brilliant discovery of Janssen 

 and Lockyer permits us henceforth constantly to follow, 



* We could assuredly manage it, but it would be necessary to have at 

 our disposal means of execution superior to the resources of a private indi- 

 vidual. 



or when science will be in a condition to approach the 

 investigation of those mysterious st.ir clusters which 

 attraction has not been able to unite into a single sun. and 

 which appear to us under forms so strange and withal so 

 geometric. 



Whatever may be the value of these experiments, it 

 is of importance, I believe, to science, not to leave this 

 beautiful question of the figure of comets without any 

 other answer than theyV ne sais of Arago, and it is of not 

 less importance to natural philosophy to prove that the 

 forces which rule the stars are none other than those 

 which act around us at the surface of the earth. If it 

 should displease any sage metaphysician that I have tried 

 to establish a duality of forces in a region where he vainly 

 flattered himself that unity reigned, I pray him to con- 

 sider that, if it is possible to transform, so to speak, 

 certain forces into each other, to produce, for example, 

 heat by means of the concussion of a body acted on by 

 terrestrial attraction, then electricity by means of this 

 heat, magnetism by means of this electricity, and finally 

 to attract a very peculiar sort of matter by means of this 

 magnetism, we have not succeeded in the least in 

 transforming the attractive force of the least molecule, 

 since its weight remains invariable through all the modi- 



