56 Dr. J. H. Gladstone on the Specific Refraction 



a very large number of its compounds, whilst mothers it may 

 exert a different influence. This has naturally attracted the 

 attention of organic chemists to the subject, and promises to 

 be a very fruitful field of investigation. 



In the present communication 1 intend to confine my remarks 

 to those groups of bodies which exhibit isomerism. It is well 

 known that two or more compound bodies, differing from one 

 another in physical or chemical properties, may be composed 

 of the same elements in precisely the same proportions. In 

 such a case the specific refraction may be the same, or it may be 

 different. 



It is found to be the same notwithstanding differences 

 of other optical properties, differences of molecular weight, 

 and differences of chemical properties. 



1st. Differences of other optical properties. — It so happens 

 that among the bodies which display the phenomena of cir- 

 cular polarization, there are several isomeric groups, and that 

 different members of these groups differ from one another in 

 their effect on the polarized ray. Thus with respect to the 

 different terpenes, C 10 H 16 , some of them rotate the plane of 

 polarization to the right and others to the left, and that to 

 different degrees ; yet they have all practically the same spe- 

 cific refraction, 0537*. 



It was also an early observation that solutions of cane-, 

 grape-, and honey-sugar, and gum, of the same strength, 

 though they differ largely in their effect on polarized light, 

 give the same, or very nearly the same, refraction and dis- 

 persionf. 



Again, it is well known that tartaric acid is an optically 

 active substance, and racemic acid optically inactive, while 

 their chemical composition, C 4 H 6 6 , is identical. The fol- 

 lowing table gives the specific refraction and dispersion of 

 these two acids in solution as deduced from data previously 

 published %. The specific dispersion is the difference between 

 the specific refraction for the line A and that for the line H, 



or, which is the same thing, ^-= — -. 





Specific 

 refraction. 



Specific 

 dispersion. 



Tartaric acid 



•3020 

 •3036 



•0131 

 •0143 



Racemic acid 





Carvol and menthol, the odorous principles of caraway and 



* Chem. Soc. Journ. 18C4, p. 18. 

 X Ibid. 1809, p. 30. 



t Phil. Trans. 1803, p. 332. 



