6 L ARMOR, Physical Aspect of the Atomic Theory. 



cant that in both cases some kind of atomism has been 

 a mental necessity, as it was in the eadiest Greek 

 inquiries. 



We have recalled that, in order to make way for the 

 principle of gravitation, Newtonians were compelled to- 

 clear the celestial spaces of the " resisting medium " which 

 constituted the aether of Descartes. But that by no 

 means implied any belief that gravitation did not require 

 a medium for its transmission. Towards the end of his 

 life, Newton allowed himself to set down formally in the 

 famous series of " Questions " appended to the second 

 edition (1717) of his "Opticks,"* his speculations on this 

 and related subjects concerning the constitution of matter^ 

 pervaded as they were by constant suggestion of the 

 vibratory motions which constitute heat, the radiation 

 which these motions excite, and their close relation to 

 chemical change. His ideas (Query 17 seq?), of essentially 

 modern type, involved a medium infinitely more rare 

 than air and of infinitely stronger elasticity — aether is his 

 own name for it — amidst the waves of which the atoms 

 of matter and the corpuscles which he took to constitute 

 light were agitated like logs in a sea ; of such a medium 

 the elastic pressure, weaker on the adjacent sides of 

 bodies, might, as he thought, in some way represent 

 gravitational attraction, while its dead resistance to 

 planets moving through it would be, owing to its small 

 mass, quite negligible. 



At the end of a prolonged physico-chemical dis- 

 cussion he sums up his atomic view of the constitution of 

 matter {loc. cit., p. 375) in archaic terms with deep modern 

 significance, that have often been quoted : 



"All these things being consider'd, it seems probable to me 

 that God in the Beginning form'd Matter in solid, massy, hard, 

 * "Opticks," ed. (3), pp. 313—382. 



