208 KINETIC THEORIES OF GRAVITATION. 



cause of gravity is what I do not pretend to know, and therefore would 

 take more time to consider of it."* 



In his third letter, dated February 25, 1G92-3, he expresses himself 

 somewhat less guardedly thus: "It is inconceivable that inanimate 

 brute matter should, without the mediation of something else which is 

 not material, operate upon and affect other matter, without mutual con- 

 tact, as it must do if gravitation in the sense of Epicurus be essential 

 and inherent in it. And this is one reason why I desired you would not 

 ascribe 'innate gravity' to me. That gravity should be innate, inher- 

 ent, and essential to matter, so that one body may act upon another at 

 a distance, through a vacuum, without the mediation of anything else, 

 by and through which their action and force may be conveyed from one 

 to another, is to me so great an absurdity, that I believe no man who 

 has in philosophical matters a competent faculty of thinking can ever 

 fall into it. Gravity must be caused by an agent acting constantly ac- 

 cording to certain laws ; but whether this agent be material or imma- 

 terial, I have left to the consideration of my readers." t 



At the conclusion of the third book of his Principia, Newton remarks : 

 " Hitherto I have not been able to discover the cause of those properties 

 of gravity from phenomena, and I frame no hypothesis ; for whatever is 

 not deduced from the phenomona is to be called an hypothesis. . . . 

 To us it is enough that gravity does really exist, and act. according to 

 the laws which we have explained." 



Still twenty-five years later than the date of these oft-quoted Bentley 

 letters, Newton again recurred to the subject in an appendix to the 

 second edition of his " Optics," published in 1717. After suggesting 

 that the chromatic dispersion of luminous rays by refraction might be 

 due to varying wave-lengths of an all-pervading " aetherial medium," 

 (as previously referred to,) he asks : " Is not this medium much rarer 

 within the dense bodies of the sun, stars, planets, and comets, than in 

 the empty celestial spaces between them ? And in passing from them 

 to greater distances, doth it not grow denser and denser perpetually, 

 and thereby cause the gravity of those great bodies toward one another, 

 and of their parts toward bodies ; every body endeavoring to go from 

 the denser parts of the medium toward the rarer f . . . And though 

 this increase of density may at great distances be exceeding slow, yet 

 if the elastic force of this medium be exceeding great, it may suffice to 

 impel bodies from the denser parts of the medium toward the rarer, 

 with all that power which we call gravity." | 



The intellectual spirit of the age in which "gravitation" was estab- 

 lished was one of strong reaction from the previous metaphysical sway 

 of " occult qualities f and that the above crude suggestion (perhaps 

 offered too much in deference to that spirit) by no means satisfied the 

 judgment of Newton, is shown by his subsequent inclination to dispense 



' Works, edited by Horsley, vol. iv, p. 437. 

 t Works, ut 8upr., vol. iv, p. 438. 

 I >ptics, book iii, appendix. Query 21. 



