301 Theories of the Rotational Optical Activity. 



theory makes it depend on the mean index of refraction of 

 the active medium by the relation (p. 826,' vol. xxv.) 



M = /,( 6 2_l) [ a ( 6 2_l) + 1 ]. 



This theory has therefore the disadvantage that it does, 

 not explain the existence of cases, as that o£ saccharose. 



for which [a>] remains constant at 9™, whilst the factor 



(e 2 — 1) (e 2 + 2) varies about - . 



Mr. Livens gives no numerical example showing the ap- 

 plication of this formula. In all cases in which the Physical 

 Tables give data permitting us to verify it, I have found it 

 inadequate. It is so with tartaric acid, since the rotator v 

 power for the D line decreases with the concentration, whilst 

 the index for this same line increases. The rotatory power 

 of the same compound in different solvents, in much diluted 

 solutions, should vary in the same way as the indices of 

 these solvents ; it is not so in aboat one case out of two, for 

 instance in the case of camphor, camphoric acid, cinchon- 

 idine. Even the example cited by Mr. Livens, of ethyl 

 tartrate, is not favourable to his theory, because the curves 

 of the form represented page 82± (vol. xxv.) can be obtained 

 only by assuming for the coefficients of the general formulae- 

 unacceptable values ; the changes of sign cannot be expected 

 by Lorentz's theory, since the quantity (e 2 — 1) [a(e 2 — 1) + 1] 

 should change its sign, which is impossible. 



Since I could not find any case of a variation of the rota- 

 tory power with the concentration and the nature of the 

 solvent of which Lorentz's theory gave a precise account, 

 I prefer Drude's theory : this theory brings us to Biot's law, 

 the considerable interest of which has been shown by such 

 researches as those of Darmois ; we may complete it, when 

 Biot's law can no longer be applied, by hypotheses as to the 

 modifications of the active molecule * or of the properties of 

 the electrons which it contains. 



Yours truly, 

 G. Bkuhat, 

 Docteur es Sciences, 

 Agrege-preparateur a l'Ecole Normale Superieure. 

 Paris, 30 juin, 1914. 



* Those modifications, to which Lorentz's theory will be driven to- 

 have recourse more often than Drude's theory, have not, I think, so small 

 an action as Mr. Livens has asserted. His reasoning regarding this 

 effect (vol. xxvi. p. 537) is based only on the constancy of the factor X 

 for all salts of the same acid or base, and for this there is not sufficient 

 evidence. 



