Vlll NOTES. 



attraction, may be performed by impulse, or by some other means unknown 

 to me. I use that word here to signify any force by which bodies tend 

 towards one another, whatsoever be the cause." 



( 41 ) p. 22. " I suppose the rarer sether within bodies, and the denser 

 without them." Operum Newtoui, Tomus iv. (ed. 1782, Sam. Horsley), 

 p. 386 ; with application to the explanation of diffraction or bending of light 

 discovered by Grimaldi. At the conclusion of Newton's letter to Robert 

 Boyle, written in February 1678, p. 394, he says, " I shall set down one 

 conjecture more which came into my mind : it is about the cause of gravity." 

 Newton's correspondence with Oldenburg, in December 1675, also shows 

 that he was not, at that period, disinclined to the hypothesis of an aether. 

 According to it the impulse of material light would put the sether in vibration; 

 the vibrations of the sether, which is akin to a nervous fluid, not by themselves 

 producing light. See, respecting the controversy with Hook, Horsley, T. iv. 

 p. 378380. 



( 4 -) p. 22. Brewster, Life of Sir Isaac Newton, p. 303305. 



( 43 ) p. 23. The precautionary explanation, " not to take gravity for an 

 essential property of matter," given by Newton in the " Second Advertise- 

 ment," contrasts with the forces of attraction and repulsion which he attri- 

 butes to all molecules, in order to explain, in a manner accordant with the 

 theory of emission, the phsenomena of refraction and reflection of rays of light 

 from mirror surfaces " before actual contact." (Newton, Opticks, Book ii. 

 Prop. 8, p. 241; and Brewster's Life of Newton, p. 301.) According to 

 Kant (Die metaphysischen Anfangsgriinde der Naturwissenschaft, 1800, 

 S. 28), the existence of matter, without these forces of attraction and 

 repulsion, cannot be imagined. According to him, therefore, as according to 

 the earlier Goodwin Knight (Phil. Trans. 1748, p. 264), all physical pheno- 

 mena are to be traced back to the conflict of these two fundamental forces. 

 In the atomic systems, which are diametrically opposed to Kant's dynamic 

 views, and according to an assumption which was widely diffused, especially 

 through the influence of Lavoisier, the attractive force is attributed to the 

 ultimate particles or molecules of which all bodies consist, and the repulsive 

 force to the atmospheres of caloric which surround the molecules. In this 

 hypothesis, which regards the so-called caloric as matter in a constant state 

 of expansion, there are assumed two different kinds of matter; i. e. two 

 different elementary substances, as in the myth of two kinds of sether. 

 (Newton, Opt. Query 28, p. 399.) Oue then asks, What is it which again 

 expands this caloric matter ? Considerations on the density of the molecules 



