592 M. chevreul's examination of an optical character 



Secondly, in relation to its probable utility in distinguishing the 

 various arrangements of tlio atoms or particles of a particular species, 

 and in estimating the alterations which may occur in bodies of deter- 

 minate species mixed together ; and to (in my apprehension) its real use 

 as a reagent or indication in the determination of chemical species of or- 

 ganic origin. 



§ 1. 

 On the objections which may be urged against the importance of the op- 

 tical character. 



Article I. — On the objections which may be urged against the import- 

 ance of the optical character in immediate chemical analysis. 



3. We examine in succession the cases ; first, in which a juice causes 

 a deviation in the plane of polarization to the left ; secondly, in whicli 

 the deviation is to the right; and lastly, in which there is no deviation. 

 We shall afterwards treat of the difficulty of estimating the quantity of 

 active matter from the density of the liquid in which it is dissolved, a 

 difficulty which arises in the two cases of deviation. 



a. Deviation to the left. 



4. When a deviation to the left in the plane of polarization is ob- 

 served, how is it to be immediately ascertained whether this property 

 belongs to gum Arabic, or to grape sugar not solidified, since it is 

 common to them both ? How are we to be certain that the property 

 of the juice proceeds from only one of these substances, and is not the 

 result of the activity of them both ? Lastly, what certainty is there 

 that it is not caused by other bodies than the gum and the grape sugar 

 not solidified ? 



b. Deviation to tlie right. 



5. There is the same uncertainty if the subject of observation be a 

 deviation to the right ; for the dextrine of Biot, cane sugar, sugar of 

 starch of the first formation, sugar of starch of the second formation, 

 and solidified grape sugar have all the property of producing a devia- 

 tion to the right in the plane of polarization. 



6. This is not all : sugar of starch of the first formation and cane 

 sugar have nearly the same energy, so that, as M. Biot acknowledges, 

 recourse must be had either to alcoholic fermentation which interverts 

 the plane of polarization of cane sugar to the left, and which leaves 

 unaltered that of sugar of fecula, or to sulphuric acid which produces 

 the same results. M. Biot gives the preference to the last method, 

 because he says that fermentation is an operation not sufficiently un- 

 derstood. But as sulphuric acid develops sugar of starch and grape 

 sugar with principles which are not saccharine, may not the employ- 



