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. 



1. M. Biot quotes a case in which he found a fiuid 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 + 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 qunntity 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 Annalen du Afuseum d'llistoire Naturelle, 3rd series, vol. ii. p. .341. 



