relative to Black, Watt, and Cavendish. 483 



as they embraced, and embodied, real facts and accurate con- 

 ceptions of phaenomena. 



The subjects on which in this point of view the above-men- 

 tioned hypotheses, taken together with the questions in the 

 Optics, threw the most important light, were the phaenomena 

 of colours, and of chemistry. I shall confine myself to his 

 speculations on the latter subject, which lead directly to the 

 question at issue — namely, what were the ideas of philosophers 

 before the time of Black respecting the nature of air, and 

 whether the unity of the aerial element was any part of their 

 belief. 



Most remarkable, among the divinations of Newton, is his 

 introduction of the doctrine of chemical affinity in the optical 

 queries, where he connects the phaenomena of chemistry 

 with those of electricity, as both due to molecular forces acting 

 at insensible distances. He enumerates electricity among those 

 " attractions which reach to sensible distances, and so have 

 been observed by vulgar eyes ;" he then suggests, that " there 

 may be others which reach to so small distances as hitherto 

 escape observation," and adding that " perhaps electrical at- 

 traction may reach to such small distances, even without being 

 excited by friction*" goes on to couple it with the phaeno- 

 mena of chemical affinity, as produced by the same species of 

 force. What is this, if it be well weighed, but the principle 

 of all that experience has since brought to light in respect to 

 galvanic and electro-chemical forces? here was the prophet's 

 eye, anticipating the progress of science and the actual indi- 

 cations of the kind of force which he surmised. 



That which follows on the point of chemical affinity itself 

 is equally remarkable. For observe how, guided in this in- 

 stance by the few obscure phaenomena before him, he deals 

 with the molecules which represent this peculiar form of at- 

 traction : they are not elementary molecules, nor molecules of 

 equal magnitude, but compound 'particles whose force of affinity 

 is in the inverse ratio of their composition — " the smallest par- 

 ticles cohering by the strongest attractions, and composing 

 bigger particles of weaker virtue, and many of these cohering, 

 and composing bigger particles whose virtue is still weaker, 

 and so on, for divers successions, until the progression end in 

 the biggest particles on which the operations of chemistry, 

 and the colours of natural bodies depend, and which by co- 

 hering compose bodies of a sensible magnitudef." Have we 

 not, in this conception of chemical affinity as depending on 

 the successive addition of units of force, the principle of multi- 

 ple proportions, of which the experimental demonstration was 

 * Optics, book iii. p. 351. t Ibid. p. 370. 



