26 COSMOS. 



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

 years afterwards, as we learn from a letter he wrote to Halley, 

 Newton entirely relinquished this hypothesis of the rarer and 

 denser a3ther. 42 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 second edition of 

 his Optics, that he did not by any means consider gravity as 

 an "essential property of bodies"; 43 whilst Gilbert, as early 



42 See Brewster's Life of Sir Isaac Newton, pp. 303-305. 



43 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, 

 \vhich he ascribes to all molecular particles, in order, according 

 to the theory of emission, to explain the phenomena of the 

 refraction and repulsion of the rays of light from reilecting 

 surfaces " without their actual contact." (Newton, Opticks, 

 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 cannot conceive the 

 existence of matter without these forces of attraction and re- 

 pulsion. All physical phenomena are therefore reduced by 

 him, as previously by Goodwin Knight (Philos. Transact. 

 1748, p. 264), to the conflict of two elementary forces. In 

 the atomic theories which were diametrically opposed to 

 Kant's dynamic views, the force of attraction was referred, in 

 accordance with a view specially promulgated by Lavoisier, 

 to the discrete solid elementary molecules of which all bodies 

 are supposed to consist ; while the force of repulsion was 

 attributed to the atmospheres of heat surrounding all element- 

 ary corpuscles. This hypothesis, which regards the so-called 

 caloric as a constantly expanded matter, assumes the existence 

 of two elementary substances, as in the mythical idea of two 

 kinds of sether. (Newton, Opticks, query 28, p. 339.) Here 

 the question arises, what causes this caloric matter to expand ? 

 Considerations on the density of molecules in comparison 

 with that of their aggregates (the entire body) lead, according 

 to atomic hypotheses, to the result, that the distance between 

 elementary corpuscles is far greater than their diameters. 



