xxiv.] EMPIRICAL KNOWLEDGE, EXPLANATION,^. 527 



than that detected by Newton concerning the h]Vh re- 

 fractive powers of combustible substances. Newton's 

 ffirti ^ ?T T e aln J St as va - ue as those Prevalent 

 inrt \ ? bS f V6d that Certain " fat > sulphureous, 

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

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

 two or three times greater than might be anticipated from 

 heir densities.' The enormous refractive index of diamondl 

 led him Wlt .h great sagacity to regard this substance as 

 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 m 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 



SnStoTfe r^rfr h f e ' in com P ari on 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 refraction The oils and hydrocarbons generally possess 



?T dlCeS 'i But aU tWs kn0wled - e remains! the 

 present day purely empirical, no connection havin" been 

 pointed out > between this coincidence of inflammability and 

 high refractive power, with other laws of chemistry or optics 

 Npwt rrt n tiCe J aS P inted ut b ^ B ter, that if 

 fnd 0?f l /? y*?*% two minerals, Greenockite 

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

 dictions would have proved false, showing sufficiently that 

 fee did not make any sure induction on the subject In 

 dpnS eSent . 7' the relat ! n f the refr active index to the 

 ?tfp y an ? atomic wei ^ ht of a substance is becoming a 

 matter of theory; yet there remain specific differences of 

 ctmg power known only on empirical grounds, and it 

 -curious that in hydrogen an abnormally high refractive 

 power has been found to be joined to inflammability. 



Ine 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 OplicJcs. Third edit. p. 249 



8ter ; Treatise on New Philosophical Instruments, p. 266, &c. 



