316 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
18. But the physical conditions which we have hitherto as- 
signed to the experiment are still very limited. When an ac- 
tive substance, solid or fluid, is dissolved in an inactive liquid, 
or mixed with it, so that a continuous liquid results, m which 
the chemical groups peculiar to each of the two bodies preserve 
- their primitive constitution without mutual decomposition, it is 
doubtless by a rare exception that these groups may be sup- 
posed to insinuate themselves amongst one another, and to dif- 
fuse themselves with uniformity throughout the whole space, 
-—_—- 
remaining wholly insensible to their mutual presence. We ought — 
rather to suppose that in general these primitive groups, or some 
of them, enter into actual combination, so as to compose mo- 
mentaneously new groups, which may act upon polarized light 
quite otherwise than those from which they are derived; and — 
this conception evidently includes, as a particular case, that of 
simple mixture, which we considered above. To prepare for- 
mulz applicable to the experiments generally, it is necessary to 
adapt them to that possible diversity of conditions. In order to 
simplify the question, I will suppose that all the systems under 
consideration are formed at a constant temperature, and that 
they are composed solely of the same active substance, succes- 
sively associated with different and progressively increasing 
doses of one and the same inactive liquid, under the sole condi- 
tion that the mixed system can subsist in a state of fluidity. In _ 
these circumstances, limited as they are, chemists are of opinion - 
that every medium thus formed may simultaneously contain 
several combinations of the two bodies, constituted in the dif- 
ferent proportions of simple multiples. Although such a diver- 
sity appears rather capable of being occasioned by the progressive 
effect of time and variable circumstances, than from a common 
and instantaneous reaction, I shall not exclude it. But I shall 
suppose that the constitution of each mixed system, whatever 
it be, remains constant whilst its optical effect is observed. 
This will be conformable to experiment for the systems which 
we shall consider, since their action upon polarized light is 
always found to be the same, when they are replaced in the same 
conditions of quantities and of temperature. 
19. Taking then any one of these systems with its actual con- 
stitution, however complex it may be supposed, it may happen 
that the quantity of the inactive liquid employed may be more 
than sufficient, or sufficient, or insufficient, to saturate com- 
