TO CHEMICAL RESEARCHES, IN REPLY TO M. CHEVREUL. 603 
origin in which I had not previously suspected its existence. I then 
resumed with renewed attention all the experiments which could serve 
as bases for establishing the details; and these results were accompanied 
by the formule necessary to deduce the comparable consequences; esta- 
blishing for each substance, whether simple or compound, what I call 
its power of actual molecular rotation, which is the angular deviation 
which it exercises upon the plane of polarization of a certain simple 
ray, with a thickness of one millemetre, and a hypothetical density 
equal to unity. Though the volume of the Academy in which these 
researches are inserted has not yet appeared before the public, I have 
sent within about the last year printed copies of them to several che- 
mists both Frenchmen and foreigners, and they have served as the 
foundation of all my subsequent researches. For I have since found 
it sufficient to apply the same methods and the same formule in the 
various experiments which I have undertaken, simply extending or cor- 
roborating them by the additional processes which the development of 
my researches required or suggested ; so that to dissipate the different 
objections that M. Chevreul has raised, at least those which I have well 
understood, I shall merely have to quote the corresponding results 
which are already published in these Annals. 
But first I shall greatly simplify this discussion by declaring that I 
have not any intention of following M. Chevreul in the most extended 
article of his dissertation, in which he examines “ the objections which 
may be urged against the importance of the optical character in the de- 
finition of chemical species.” Having never proposed its application to 
such a use, I have not to defend it upon this point ; more especially as 
in my own opinion no character taken separately is sufficient to define, 
I will not say a chemical species in general merely, but even a substance 
individually unique. Such definitions are and can be merely the ex. 
pression of our ignorance ; or in other terms, of our actual knowledge. 
An attempt was made to class natural solid bodies according to their 
crystallization ; but among them were found some rigorously isomor- 
phous ; for instance, those which crystallize in cubes or in regular oc- 
tahedrons proved to be such by the complete symmetry of their derived 
forms. A second attempt was made to class them according to che- 
mical composition: this was defeated by the discovery of bodies exactly 
isomeric. ‘These two examples may suffice to convince us that the de- 
finition of bodies should be established upon the union of the obser- 
vable characters that each of them possesses; and that this definition 
must always be merely provisional, as another system of material par- 
ticles may be discovered tomorrow, possessing in common the whole 
of this first collection of properties. The character derived from cir- 
cular polarization is therefore, and can be, nothing more than an addi- 
tional element, a new condition of the actual molecular state of the 
