xxiv.] EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 527 



than that detected by Newton concerning the high re- 

 fractive powers of combustible substances. Newton's 

 chemical notions were almost as vague as those prevalent 

 in his day, but he observed that certain "fat, sulphureous, 

 unctuous bodies," as he calls them, such as camphor, oils 

 spirit of turpentine, amber, &c., have refractive powers 

 two or three times greater than might be anticipated from 

 their densities. 1 The enormous refractive index of diamond, 

 led him with great sagacity to regard this substance as 

 of the same unctuous or inflammable nature, so that he 

 may be regarded as predicting the combustibility of the 

 diamond, afterwards demonstrated by the Florentine 

 Academicians in 1694. Brewster having entered into a 

 long investigation of the refractive powers of different 

 substances, confirmed Newton's assertions, and found that 

 the three elementary combustible substances, diamond, 

 phosphorus, and sulphur, have, in comparison with their 

 densities, by far the highest known refractive indices, 2 and 

 there are only a few substances, such as chromate of lead 

 or glass of antimony, which exceed them in absolute power 

 of retraction. The oils and hydrocarbons generally possess 

 excessive indices. But all this knowledge remains to the 

 present day purely empirical, no connection having been 

 pointed out between this coincidence of inflammability and 

 high refractive power, with other laws of chemistry or optics. 

 It is worth notice, as pointed out by Brewster, that if 

 Newton had argued concerning two minerals, Greenockite 

 and Octahedrite, as he did concerning diamond, his pre- 

 dictions would have proved false, showing sufficiently that 

 he did not make any sure induction on the subject. In 

 the present day, the relation of the refractive index to the 

 density and atomic weight of a substance is becoming a 

 matter of theory; yet there remain specific differences of 

 refracting power known only on empirical grounds, and it 

 is curious that in hydrogen an abnormally high refractive 

 power has been found to be joined to inflammability. 



The science of chemistry, however much its 'theory may 

 have progressed, still presents us with a vast body of em- 

 pirical knowledge. Not only is it as yet hopeless to attempt 



' Newton's Opticks. Third edit. p. 249. 



2 Brewster, Treatise on New Philosophical Instruments, p. 266, && 



