594 M. CHEVREUL’S EXAMINATION OF AN OPTICAL CHARACTER 
10. Without such researches, without accurate knowledge of the 
nature of the bodies which accompany one or several active principles, 
of their proportions relative to their solvents, and of their influence 
upon the density of the juice in which they are contained, how can a 
general rule be formed for the appreciation of the density proper to the 
active principle or principles of a vegetable juice, endowed with a pro- 
perty of causing a deviation in the plane of polarization, in order to 
deduce from it the nature and the proportions of the principle or prin- 
ciples? Now science is actually deficient in all the investigations neces- 
sary to surmount this difficulty. 
ArTIcLe Il.—On the objections which may be urged against the import- 
ance of the optical character in the definition of chemical species. 
11. Though there is not for the classification and definition of che- 
mical species, a subordination of characters comparable to that observed 
in natural classifications, among which those endowed with life, such as 
the zoological and botanical species, are ordinated into genera, families, 
orders, and superior divisions ; yet there are properties in chemistry, 
the importance of which rest upon distinctions essential to that science, 
which furnish characters more or less rational, for grouping the species 
together and distinguishing them from each other. The differences to 
be remarked between the classification of chemical species and that of 
zoological and botanical species arise principally, as I have elsewhere 
shown, from the small number of general properties which are adapted 
to serve as general characters of chemical species, from the corre- 
lative relation of these properties, and from the circumstance that the 
special object of chemistry is the circumscription of the species. We 
will now enter upon a few details relative to the properties capable of 
serving as characters of chemical species, whether for grouping them 
or for distinguishing each of them in particular, and supplying means 
by which they may be known. 
12. The combustible and comburent properties of simple bodies, and 
‘the acid and alkaline properties of compound bodies, are general pro- 
perties of such a nature, that if in consequence of their correlation, 
they will not serve for the formation of groups perfectly circumscribed, 
yet they serve to give a precise idea of a body which possesses one of 
these properties in a certain degree of energy. We will take for ex- 
ample the acidity of an oxygenated body when it is sufficiently ener- 
getic to remove the alkali from the red substance of litmus. 
Since it possesses this property which gives it the function of an acid, 
it may be concluded : 
Ist. Tha! it will enter into combination with all, or at least with the 
greater part of the compounds of an alkaline quality. 
