54 • On the UNEQUAL 



the marine acid, does not difperfe the red and violet rays quite 

 fo much. The green ray, as in the marine acid, is found 

 among the more refrangible rays ; but it approaches nearer to 

 the place of the mean refrangible ray in the nitrous acid than 

 in the marine. The green ray is alfo nearer to the place of the 

 mean refrangible ray, than it is in efTential oils or in faturated 

 metallic folutions ; and therefore the nitrous acid appears by 

 thefe experiments to difperfe the feveral orders of rays more 

 nearly in the fame proportion in which crown glafs does, than 

 any uncompounded difperfive medium, and would, I have no 

 doubt, do fo exactly, if flightly impregnated with mercury, 

 though this I have not tried. 



The vitriolic acid is fcarcely to be clafTed among difperfive 

 mediums. The following experiment is the lad I made on the 

 fubjecl. In a very good object-glafs, of that kind before de- 

 fcribed, in which fpirit of wine is one of the mediums em- 

 ployed, I fubftituted fuccefTively for this fpirit the vitriolic 

 acid and a folution of fixed alkaline fait, both of them of near- 

 ly the fame mean refractive denfity as the fpirit of wine. Thefe 

 three fluids, although they differ fo widely in their chemical 

 properties, have their optical properties fo nearly alike, that I 

 found it difficult to determine which was the medium em- 

 ployed. For when the fecondary colour is not corrected, as 

 was the cafe in this object-glafs, the change of colour produced 

 in the green and purple fringes to render it apparent, muft be 

 confiderable, a flight fhade of difference not being eafily di- 

 flinguifhable.' I therefore repeated the trial with an object- 

 glafs, in which this green and purple light is totally removed •; 

 and then both the vitriolic acid, and the folution of fixed al- 

 kali, when of equal mean refractive denfity with fpirit of wine, 

 appeared very fenfibly more difperfive than the fpirit. The 

 difference in this refpect between the acid and the alkali was 

 fcarcely to be diftinguifhed ; and the effect of a folution of 

 cauftic alkali appeared to be nearly the fame as that of mild 



alkali 



