INTRODUCTION. 23 



result of some higher and still unknown power, or of " the 

 centrifugal force of the aether, which fills the realms of space, 

 and is rarer within bodies, but increases in density outward. 

 The latter view is set forth in detail in a letter to Robert 

 Boyle* (dated February 28, 1678), which ends with the 

 words, " I seek the cause of gravity in the sether." Eight 

 years afterward, as we learn from a letter he wrote to Hal 

 ley, Newton entirely relinquished this hypothesis of the rarer 

 and denser sether.f It is especially worthy of notice, that 

 in 1717, nine years before his death, he should have deemed 

 it necessary expressly to state, in the short preface to the sec- 

 ond edition of his Optics, that he did not by any means con- 

 sider gravity as an " essential property of bodies ;"J while 



two or three general principles of motion from phenomena, and after- 

 ward to tell us how the properties and actions of all corporeal things 

 follow from those manifest principles, would be a very great step in phi- 

 losophy, though the causes of those principles were not yet discovered ; 

 and therefore I scruple not to propose the principles of motion, and leave 

 their causes to be found out." Newton's Optics, p. 377. In a previ- 

 ous portion of the same work, at query 31, p. 351, he writes as follows : 

 " Bodies act one upon another by the attraction of gravity, magnetism, 

 and electricity ; and it is not improbable that there may be more at- 

 tractive powers than these. How these attractions may be performed 

 I do not here consider. What I call attraction may be performed by 

 impulse, or by some other means unknown to me. I use that word 

 lio <; to signify only in general any force by which bodies tend toward 

 on^ another, whatsoever be the cause." 



* " I suppose the rarer sether within bodies, and the denser without 

 them." Operum Newtoni, tomus iv. (ed. 1782, Sam. Horsley), p. 386. 

 The above observation was made in reference to the explanation of the 

 discovery made by Grimaldi of the diffraction or inflection of light. At 

 the close of Newton's letter to Robert Boyle, February, 1678, p. 94, he 

 says : " I shall set down one conjecture more which came into my mind: 

 it is about the cause of gravity. . . ." His correspondence with Olden- 

 burg (December, 1675) shows that the great philosopher was not at 

 that time averse to the " aether hypotheses." According to these views, 

 the impulse of material light causes the aether to vibrate ; but the vibra- 

 tions of the sether alone, which fias some affinity to a nervous fluid, does 

 not generate light. In reference to the contest with Hooke, consult 

 Horsley, t. iv., p. 378-380. 



t See Brewster's Life of Sir Isaac Newton, p. 303-305. 



t Newton's words " not to take gravity for an essential property of 

 bodies" in the " Second Advertisement" contrast with his remarks on 

 the forces of attraction and repulsion, which he ascribes to all molecu- 

 lar particles, in order, according to the theory of emission, to explain 

 the phenomena of the refraction and repulsion of the rays of light from 

 reflecting surfaces "without their actual contact." (Newton, Optict, 

 book ii., prop. 8, p. 241, and Brewster, Op. cit., p. 301.) According 

 to Kant (see Die Metaphysischen Anfangsgrunde der Naturwissenschaft, 

 1800, s. 28), we can not conceive the existence of matter without these 

 forces of attraction and repulsion. All physical phenomena are there- 



