482 Rev. W. V.Harcourt on Lord Brougham's statements 



sumed greater repulsion of the larger particles of aether from 

 their pores. In this letter he made a comprehensive conjectural 

 effort to reduce the whole system of the laws of nature, 

 whether bearing the aspect of impulse or attraction, under the 

 dominion of two kinds of repulsive force, the one of mutual 

 repulsion between the particles of {Ether, the other of repul- 

 sion between the particles of aether and those of ordinary 

 matter. 



In the edition of his Optics which he printed nearly forty 

 years afterwards, in 1717, he deliberately delivered, when in 

 full possession of the laws of gravity, another hypothesis on 

 this subject, taking for his fundamental assumption this fact 

 presumed from the phsenomena of light, that a subtle and 

 elastic fluid, within bodies and without them, follows some law 

 of density which increases from their centre indefinitely into 

 space, and merely representing the force by which they gra- 

 vitate as repulsive. Further he has not explained himself ; and 

 it may perhaps be inferred from his subsequently omitting in 

 an edition of the Principia the mention of gravity when he 

 enumerates, at the end of that work, the other phaenomena of 

 molecular attraction and cohesion, electricity, light, heat, mus- 

 cular motion, and nervous sensation, which he attributes to 

 the force of "a very subtle spirit," pervading and lurking in 

 dense bodies, but not yet sufficiently manifested by experi- 

 ments, — that he was dissatisfied with his own conceptions of 

 its gravific action, and had never reduced them into a mathe- 

 matical form. Thus much however it may be worth while to 

 remark, as deserving perhaps the attention of" those who may 

 follow Vince and Playfair in discussing the possible sufficiency 

 of Newton's hypothesis — that in the Optics he alleges reasons 

 for supposing the elastic force of aetherial particles to be in- 

 versely proportional to their magnitude*. This leaves ground 

 to believe that with the supposition of a density increasing with 

 the distance he may have combined his former conjecture of an 

 increasing magnitude of the particles, and so far an elasticity 

 proportionably diminished ; which gives latitude at least to 

 the hypothesis, as making the mutual repulsion of the parti- 

 cles at different distances from the centre, depend on more 

 elements than one. 



But the knowledge of the experimental laws of molecular 

 force was not sufficiently advanced to justify any serious at- 

 tempt at mathematical theory, either on this subject or any 

 other connected with them ; nor did he offer these hypotheses 

 as more than cursory hints, and specimens of a generalising 

 and simplifying spirit of conjecture, so far illustrating nature, 

 * Optics, ed. 4. book iii. p. 326. 



