160 THE PRINCIPLES OF SCIENCE. 



the three elementary combustible substances, diamond, 

 phosphorus, and sulphur, have by far the highest re- 

 fractive indices known in proportion to their densities c , 

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

 lead or glass of antimony, known to exceed them in ab- 

 solute power of refraction. The oils and hydrocarbons 

 generally possess an excessive index. But this knowledge 

 remains to the present day purely empirical, no connexion 

 having been pointed out between this coincidence of in- 

 flammability and high refractive power, with other laws of 

 chemistry or optics. It is worthy of notice, however, 

 as pointed out by Brewster, that if Newton had argued 

 concerning two minerals, Greenockite arid Octahedrite, as 

 he did concerning diamond, his predictions would have 

 proved false. 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 refractive power known only 

 on empirical grounds, and it is curious that in hydrogen 

 also 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 

 empirical knowledge. Not only is it at present hopeless 

 to attempt to account for the particular group of qualities 

 belonging to each element, but there are multitudes of 

 particular facts of which no further account can be given. 

 Why should the sulphides of many metals be intensely 

 black 1 Why should a slight amount of phosphoric acid 

 have so great a power of interference with the crystalliza- 

 tion of vanadic acid d . Why should the compound silicates 

 of alkalies and alkaline metals be transparent ? Why 

 should gold be so highly ductile, and gold and silver the 



c Brewster, ' Treatise on New Philosophical Instruments/ p. 266, &c. 

 d Roscoe, Bakerian Lecture, 'Philosophical Trans.' (1868), vol. clviii. p. 6. 



