IN STUDYING QUESTIONS OF CHEMICAL MECHANICS. 337 
is what is commonly called barley-sugar. 'These plates, tra- 
- versed by a polarized ray, exercise upon it the rotatory power, 
with an energy in each case proportioned to their thickness, or 
as they interpose a greater total thickness in the course of the 
ray, when placed one after another. This power is directed 
towards the right of the observer, like that of the primitive 
sugar in solution in water. To render the observation exact, I 
poured out the syrup and cooled it briskly in a little rectangular 
vessel terminated by thin glass sides with parallel faces, the 
intervals of which had been carefully measured in the two direc- 
tions of the rectangle*. The deviations produced on the same 
simple ray are found exactly proportional to the thicknesses 
which these intervals comprise, as the apparent homogeneity of 
the mass would lead us to presume. The absolute valuation of 
the power of this mass required the knowledge of its density. 
In order to obtain it, I filled with the same solidifiable syrup a 
small phial, having first ascertained its weight and that of the 
water which it contained at a known temperature. Having then 
weighed it again, after this solidification, and deducted its own 
weight, the remainder compared to the weight of the water gave 
me the density of the solid syrup, which I found to be 1°5092. 
With this number taken for 8, and the deviations a observed on 
the red ray through the thicknesses /, I calculated the molecular 
power of the solid mass on this same ray, by the formula (1.) 
of § 8; and calculating it for 100™™, I found 
[4], = 75 = + 42°568. 
This being done, I took a known weight P of this same sugar, 
and dissolved it in nearly an equal weight E of distilled water, 
so that its ponderable proportion < in the solution was 0°50075. 
The density ¢ of the mixed system thus formed was found to be 
1:22676. With these numbers, and the deviation « produced 
by this system on the red ray through a tube / of a known length, 
I calculated the molecular power of the dissolved sugar by the 
formula (2.) of § 14, considering it as in the state of simple mix- 
__* The experiment to which I here refer is given in my Mémoire sur la 
Polarisation circulaire, inserted in tome xiii. of the Mémoires de I’ Académie 
des Sciences, p. 128 et seg. The numbers are the same as I employ here, only 
they are there discussed in an inverse manner. At page 119 of the same 
memoir will also be found the experiments on solutions of pure cane-sugar, 
mentioned in the subsequent part of the reasoning. I avail myself of this 
occasion to correct the error of a word at page 131 of that memoir, line 10 from 
the bottom: instead of plus fortes read plus faibles, 
