and Dispersion of Isomeric Bodies. 



59 



In my notebook I find the following instances of isomeric 

 bodies which differ in specific refraction : — 





Specific 

 refraction. 



Specific 

 dispersion. 



Refraction- 

 equiyalents. 





•4420 

 •4402 

 •4734 



•0207 

 •0191 



•0275 



2564 

 51-06 

 27-45 



Butyric ether, C 6 H 12 2 



Allyl alcohol, C 3 H 6 





Carvol, 10 H 14 O 



Thymol, „ 



•5122 

 •5206 



•0355 



76-83 

 78-09 



Nitraniline (a), C 6 H 6 (N0 2 )N 

 (0), 



•469 

 •546 





64-72 

 75-34 



Cresy lie acetate, C g H 10 O 2 



Benzy lie acetate, „ 



•4677 

 •4945 



•0348 

 •0394 



7015 

 74-17 



Phenyl-ethyl acetate, C 10 H 12 O 2 



Hyclrocinnamene acetate, ,, 



•4776 

 •5183 



•0382 



78-32 

 85-00 



Acetone and ally lie alcohol have been compared already by 

 Briihl, with similar results, and form one of the striking cases 

 on which his theory depends. The refraction-equivalent of 

 thymol being 6*8 above the normal, is confirmatory of the 

 position which is now given to it in the great aromatic 

 group — a position which I was disposed to assign to it ten 

 years ago, on the evidence of its optical properties. 



The two nitranilines were lent to me by Dr. Mills. Unfor- 

 tunately they are very sparingly soluble bodies ; and the 

 above figures, though each of them the mean of three obser- 

 vations, are only given as approximately correct. But that 

 they are really different there can be no doubt. 



The two pairs of acetates were lent by Dr. Hodgkinson; and 

 the last one is of particular interest, on account of the cinna- 

 mene compound having a higher refraction-equivalent than 

 can be accounted for by any existing theory. 



Specific Dispersion. 



Though there are several sources of error in determining 

 indices of refraction for the line A or for the line H, the actual 

 distance between these two may be measured with great 

 exactness, and the experimental error rarely, I believe, exceeds 

 0*0002. The possible error in the determination of the density 

 at the same temperature has also to be considered with regard 

 to specific dispersion ; but any inaccuracy in the figures above 

 given will certainly be confined to the fourth place of 

 decimals. 



Among the conclusions that may be drawn in regard to 

 dispersion are the two following : — 



1st. Where the carbon atoms are exerting a more than 

 normal influence on the refraction of the rays of light, there is 



