KINETIC THEORIES OF GRAVITATION. 207 



also of all natural bodies, whose parts are like so many fragments of 

 such plates. These seem to be most plain, genuine, and necessary con- 

 ditions of this hypothesis. And they agree so justly with my theory, 

 that if the auimadversor think lit to apply them, he need not on that 

 account apprehend a divorce from it."* 



This passage is interesting as being the earliest presentation of a 

 theory of color, now universally adopted. The same views were re- 

 peated as a suggestion, some forty-five years later, in the second edition 

 of his treatise on " Optics."t 



In his ^'Letter to the Hon. Mr. Boyle," dated February 28, 1078-9, 

 (about six years later,) Xewton, after proposing as an explanation of the 

 phenomena of cohesion, chemical affinity, &c., the "supposition" that 

 an exceedingly elastic subtile letherial substance is diffused through all 

 places and bodies, but much rarer within and near gross bodies than 

 beyond them, adds toward the conclusion of his letter: "I shall set 

 down one conjecture more, which came into my mind now as 1 was 

 writing this letter : it is about the cause of gravity. For this end I 

 will suppose aither to consist of parts differing from one another in sub- 

 tilty by indefinite degrees, ... in such a manner that from the 

 top of the air to the surface of the earth, and again from the surface of 

 the earth to the center thereof, the aether is insensibly finer and finer. 

 Imagine now any body suspended in the air or lying on the earth, and 

 the £ether being by the hypothesis grosser in the pores which are in the 

 upper parts of the body than in those which are in the lower parts, 

 and that grosser aither being less apt to be lodged in those pores than 

 the finer sether below, it will endeavor to get out, and give way to the 

 finer a3ther below, which cannot be without the bodies descending to 

 make room above for it to go into. From this supposed gradual sub- 

 tilty of the parts of the aether, some things above might be further illus- 

 trated and made more intelligible. . . . For my own part, I have 

 so little fancy to things of this nature, that had not your encourage- 

 ment moved me to it, I should never I think have thus far set pen to 

 ])aper about them." | It will be seen from the above that Newton had 

 not at this time (only three years before the crowning epoch of his life) 

 extended his conception of " gravity" to the outlying universe. 



Fourteen years later — a decade after his culminating work — this topic 

 was again incidentally touched upon by Newton in four letters ad- 

 dressed to Doctor Bentley, "containing some arguments in proof of a 

 Deity." In his second letter, dated January 17, 1692-3, he says in re- 

 ply to one from Bentley : " You sometimes speak of gravity as essential 

 and inherent to matter. Pray do not ascribe that notion to me, for the 



* Philosophical Transactions of the Royal Society ; November 18, 1672. No. 88, vol. 

 vii, p. 5088. 



t Newton's Optics. Second edition, 1777. Book iii, appendix. Query 13. 



t The Works of Isaac Newton, edited by Samuel Horsley : In 5 vols., quarto. Vol. iv, 

 pp. 335-394. 



