58 Dr. J. IT. Gladstone on the Specific Refraction 





Specific 



rclVaction. 



Specific 

 dispersion. 



Propyl iodide, 0,1] -I 



•2844* 



•2883* 



•0216 



Isopropvl iodide, ,, 





Cresol from thymol, C-II 8 



•5116 



•5091 

 •5009 



•0454 

 •0452 

 •0415 



Metacresol, „ 







Nitrobenaoic acid (a), C 7 H-(NO.,)0., 



» m ' » " " 



•3994 

 •4004 





Benzyl ie butyrate, C u H u 0i O 



•4777 

 •4S05 



•0332 

 •0331 



,, isobutyrate, ,, 





Monocliloro-toluol, CH-Cl 



•4807 

 •4836 



•0409 

 •0409 



Benzyl chloride, ,, 





Though identity of ultimate composition will generally pro- 

 duce identity of specific refraction, there are certain cases in 

 which isomeric (or, rather, metameric) bodies differ widely in 

 their power of retarding the transmitted ray. The earliest of 

 those observed was the case of aniline and picoline, C 6 H 7 N ; 

 and as Dr. Thorpe has kindly lent me his specimen of picoline, 

 I am able to confirm the previous determinations. 





Specific 

 refraction. 



Specific 

 dispersion. 





•550 

 •513 

 •522 



•0635 

 •0448 

 •0431 



Picoline {C. G. Williams) ... 





The difference in this pair was originally attributed to the 

 fact that the two bodies " are constructed very differently;" 

 and in subsequent papers it was pointed out that, when the 

 atoms of carbon were not saturated in the usual way, there is 

 an augmentation in their refractive power, analogous to what 

 happens when an element such as iron changes its valency f. 

 Briihl has expressed this more definitely by putting forward 

 the theory that, whenever two carbon atoms are doubly linked, 

 there is an increase of the refraction-equivalent amounting to 

 2*0 for the limit of the spectrum, as reckoned by Cauchy's 

 formula, or about 2' 2 for the hydrogen-line a, and of course a 

 little less than 2*2 for the line A. I have no doubt that both 

 Briihl himself and other chemists will have much to say in the 

 future as to the extent to which this theory may be in accor- 

 dance with present views of the constitution of the various 

 organic compounds. 



* Briihl gives the values 0-2873 and 0-2907 respectively for the line C. 

 t Phil. Trans. 1863, p. 338 \ Chem. Soc. Journ. 1870, p. 150 ; Proc 

 Royal Instit. March 1877. 



