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 formulae necessary to deduce the comparable consequences; esta- 

 blishing for each substance, whether simple or compound, what I call 

 its power of actual molecular rotation, wliich 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 formula 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 cox-responding 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 ivhich 

 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 



