TO CHEMICAL RESEARCHES, IN REPLY TO M. CHEVREUL. 605 



mates from which cane sugar has hitherto been extracted, those of 

 beet-root, parsneps, carrots, and marsh-mallows, presented a rotation to 

 the right, whilst all those which yield only grape sugar invariably 

 presented a rotation to the left ; thus by the word immediately I 

 meant instantly, at the very moment; and indeed in my first ob- 

 servations, I did not seek for other means of distinguishing the 

 two kinds of sugar in question, not having at that period met with 

 them naturally mixed sufficiently to conceal or intervert their 

 proper rotation. Now if it be the word immediately which has 

 shocked M. Chevreul, as expressing the pretence on my part of 

 employing solely the optical character, to the exclusion of all other, 

 and particularly of chemical means, 1 would beg him to observe that 

 I have never acted in a manner to justify this interpretation. For even 

 in my first fundamental memoir read before the Academy on the 5th 

 of October, 1832, I determined the opposite rotations of the two prin- 

 ciples of honey, the crystallizable and the uncrystallizanle, after having 

 separated them by means of alcohol ; and I have never since neglected 

 to seek all the assistance that chemistry is capable of affording. It is 

 however, I repeat, with much hesitation that I attribute to M. Chevreul 

 the suggestion of a difficulty which appears to me to be purely gram- 

 matical ; for if such were his thought, he could not, without a degree 

 of injustice of which I believe him incapable, cite my original expres- 

 sions as he has done, without adding that all my researches subsequently 

 published contradict the idea of exclusion which this interpretation at- 

 tributes to me ; and that I even formally expressed the contrary prin- 

 ciple at the commencement of my memoir on the analysis of vegetation 

 in the Gramineas, as may easily be seen. As to the rest, it will at least 

 be evident from this discussion, that neither am I who have invented 

 and applied the optical character, nor is M. Chevreul who examines it, 

 of opinion that it should be separated from the chemical characters 

 which may aid in its applications ; and this I apprehend is the only sci- 

 entific point of interest at present. 



I now arrive at the last of M. Chevreul's objections, objection d, 

 which is expressed in these terms: " Difficulty of estimating the quan- 

 tity of an active piiuciple from the density of the liquid by tvhich it is 

 held in solution." I cannot possibly understand how, or in what re- 

 spect this objection can be applied to my formulae, or to the results 

 which I have deduced from them. And certainly it is the intention of 

 the writer so to apply it ; for in his development of it, mentioning the 

 necessity for distinguishing the proportion of the active substance in the 

 solvent, in order to decide upon its specific nature, M. Chevreul in- 

 quires (p. 593.) how this proportion is to be ascertained ; and he adds 

 " it is, according to M. Biot, hy taking the densities of the liquids" a 

 method which appeared to him, and with truth, to be of difficult em- 



