; 
’ 
FOR DISTINGUISHING SACCHARINE JUICES. 593 
ment of it, at least in some instances, be productive of error? In 
short, since it is necessary to have recourse to fermentation or sulphuric 
acid, it is evident that the character of circular polarization does not 
furnish the means of immediately distinguishing the vegetable juices 
which yield sugar analogous to cane sugar, and those which only yield 
grape sugar ; and that therefore it has not the advantage of giving a 
more precise indication in organic analysis than that furnished by the 
chemical processes, which are liable to the objection of disturbing the 
equilibrium of the elements of substances, which by their means have 
been separated from each other. 
c. Case in which there is no deviation. 
7. M. Biot quotes a case in which he found a fluid of an extremely 
saccharine quality without rotation*, because it contained at the same 
time grape sugar solidified and not solidified. He remarked that time 
produces an alteration in the solidified sugar, gradually diminishing its 
property of rotation to the left and directing it towards the right; thus 
the same body spontaneously experiences a molecular alteration which 
has a tendency to cause it to pass successively through a series of 
states marked by the signs + 0 and —. After such a result, how 
is it possible to imagine that the extreme states distinguished by the 
signs + and — could be precise characters for other bodies proper to 
cause their immediate recognition in the juices of plants ? 
‘d. Difficulty of estimating the quantity of an active principle from the 
density of the fluid by which it is held in solution. 
8. The action of deviating from the plane of polarization whether to 
the left or to the right, being the product of all the active molecules 
contained in the liquid upon which the experiment is performed, it 
follows that in the most simple case, that in which the activity emanates 
from only one principle, when we would determine the specific nature 
of this principle, it will be necessary to attend to its proportion relative 
to the solvent; for as quantity may compensate for the feebleness of 
‘the action, two solutions may have the same rotatory power, though one 
‘may contain a principle much less energetic than the other. 
9. How is this proportion to be ascertained? According to M. Biot, 
by taking the density of the liquids; but if positive results can be 
‘drawn from the determination of the density, it can only be when tables 
of the respective solutions of each active principle have previously been 
formed, in each of which the densities correspond to determined pro- 
portions of the principle dissolved, and to the rotatory powers of solu- 
_ tions made according to the same proportions. 
* Nouvelles Annales du Muséum d’ Histoire Naturelle, 3rd series, vol. ii. p. 341. 
