80 DISSERTATION SECOND. [partii. 



velocity has its own value, which continues the same while 

 the transparent medium is the same. 



Light, in consequence of these views, became, in the 

 hands of Newton, the means of making important discoveries 

 concerning the internal and chemical constitution of bodies. 

 The square that is added to that of the perpendicular veloci- 

 ty of light in consequence of the attractive force of the trans- 

 parent substance, is properly the measure of the quantity of 

 that attraction, and is the same with the difference of the 

 squares j>f the velocities of the incident and the refracted 

 light. This is readily deduced, therefore, from the ratio of 

 the angle of incidence to that of refraction ; and when this is 

 done for different substances, it is found, that the above 

 measure of the refracting power of transparent bodies is 

 nearly proportional to their density, with the exception of 

 those which contain much inflammable matter in their com- 

 position, or sulphur as it was then called, which is always 

 accompanied with an increase of refracting power. 1 



Thus the refracting power, ascertained as above, when 

 divided by the density, gives quotients not very different 

 from one another, till we come to the inflammable bodies, 

 where a great increase immediately takes place. In air, 

 for instance, the quotient is 5208, in rock-crystal 5450, 

 and the same nearly in common glass. But in spirit of wine, 

 oil, amber, the same quotients are 10121, 12607, 13654. 

 Newton found in the diamond, that this quotient is still 

 greater than any of the preceding, being 14556. 2 Hence 

 he conjectured, what has since been so fully verified by 

 experiment, that the diamond, at least in part, is an inflam- 

 mable body. Observing, also, that the refracting power of 

 water is great for its density, the quotient, expounding it, as 

 above, being 7845, he concluded, that en inflammable sub- 



1 Newton's Optics, Ibid. 2 Ibid. 



