IN STUDYING QUESTIONS OF CHEMICAL MECHANICS. 311 
always to present a constant value, when the physical conditions 
taken as basis of the reasoning have been realized ; that is to 
say, when the active groups shall only have diffused themselves 
in the inactive medium without being chemically modified by it, 
and also without modifying the primitive inactivity which the 
latter is supposed to have for light ; or again, if the combination 
which may be presumed to take place between the two sub- 
stances is sufficiently feeble that these modifications may be in- 
appreciable, which would greatly extend its applicability. When 
the active substance is observed isolated, E becomes null, which 
renders ¢ equal to 1, and the expression of [a] is reduced to the 
preceding (1.) of § 8, as should be the case from the identity of 
circumstances to which the experiment is reduced. This expres- 
sion of [«], given by the equation (2.), thus reproduces more 
generally the molecular or specific rotatory power of bodies than 
the equation (1.), which supposed the active substance observed 
isolated in the liquid state. 
15. I will relate, as an example, a very simple application 
which confirms the justness of association of all its elements. 
Take two cylindrical tubes, A, B, the lengths of which are /, and 
I,, the latter larger than the former: A is filled with an active 
liquid, and the same length /, is poured into the tube B, which 
is then filled with an inactive liquid, whose chemical action on 
the other is null or insensible. We then observe A, which pro- 
duces the deviation «, on the type ray; then B, which produces 
the deviation «, on this same ray. The experiment always gives 
@,=,. This follows from the formula (2.). 
To prove this, let us take 0,, the proper density of the active 
liquid in the circumstances in which it is employed. Since it 
_ produces the deviation «, through the thickness /,, when it is 
observed alone, by designating by [«],, its proper molecular 
power, we shall have, by the formula (1.) or by the formula (2.) 
applied to these circumstances, 
ou & 
[«], = 1,8," 
Let P, be the absolute weight of this same active liquid occupy- 
ing the length /, in the tube B, and V, its volume. We shall 
evidently have 
P,=V,45 
to this weight P, a certain weight P, of an inactive liquid is 
added, which fills the tube B. Let V, be the total volume of 
