and Dispersion of Isomeric Bodies. 



57 



mint respectively, are very definite substances, of the compo- 

 sition C 10 H 14 ; but the first rotates the plane of polarization 

 very powerfully to the right, and menthol very powerfully to 

 the left; yet they have practically the same specific refraction *. 





Specific 

 gravity. 



Specific 

 refraction. 



Specific 

 dispersion. 





0-9530 

 0-9562 

 09394 



•5126 

 •5115 

 •5100 



•0362 

 •0348 

 •0331 









2nd. Differences of molecular iveight. — Interesting cases of 

 this are to be found among the essential oils, as shown in the 

 subjoined table, in which the numbers given for the terpenes 

 are the mean of ten different specimens, for the citrenes of 

 twelve specimens, and for the cedrenes of eight specimens, 

 each group probably consisting of several isomerides f . 





Specific 

 gravity. 



Specific 

 refraction. 



Specific 

 dispersion. 



Refraction- 

 equivalent. 



Terpenes, C 10 H 16 



Citrenes, C 10 H 16 



Cedrenes, C 15 H 24 



Colophene, C 20 H 32 



0-8600 

 08466 

 0-9166 

 0-9391 



•5370 

 •5475 

 •5392 

 •5413 



■0302 

 0336 

 •0307 

 •0329 



73-03 



74-46 



110-00 



147-23 



The terpenes and citrenes show a constant though not a 

 large difference in specific refraction ; but the polymeric 

 bodies of the same series seem to have so nearly the same 

 specific refraction as the terpenes that the differences may be 

 attributed to impurities or experimental error. The refraction- 

 equivalents of the three groups are of course very nearly in the 

 proportions of 2, 3, and 4. 



3rd. Differences of chemical properties. — In the cases already 

 mentioned it is possible that the differences of physical pro- 

 perties depend upon some difference in the arrangement of 

 the ultimate atoms ; but there are other cases in which this is 

 perfectly well known to be the case. Isomeric bodies of this 

 nature are more strictly termed metameric. Delffs showed, 

 as far back as 1850|, that such pairs as formic ether and acetate 

 of methyl, both of the ultimate composition C 3 H 6 2 , had the 

 same refraction ; whilst the papers of Landolt, and the more 

 recent ones of Briihl § , abound in instances. Among the 

 observations in my notebook, hitherto unpublished, occur the 

 following : — 



* Cliem. Soc. Journ. 1872, p. 9. 

 X Pogg, Ann. lxxxi. p. 470. 



t Chem. Soc. Journ. 18G4, p. 18. 

 § Liebig's Ann. cciii. pp. 1, 255. 



