IN STUDYING QUESTIONS OF CHEMICAL MECHANICS. 329 
of the planes of polarization, to which we apply them, are pro- 
duced solely by the individual action of the chemical groups 
which constitute the bodies in which they are observed, with- 
out the actual state of aggregation of these groups exercising 
any influence, either by modifying the optical power which 
they would exert if they were isolated, or by producing in their 
union effects of polarization which would imitate those, or 
would mask them by combining with them. These specifications 
are indispensable ; for experience shows that phzenomena of de- 
viation, analogous or similar to the eye to those above defined, 
may be produced in systems which do not act molecularly, But 
then they result from the natural or artificial mode of apposition 
which the sensible parts composing these systems have amongst 
themselves, and they no longer take place when disaggregated, 
On the other hand, material molecules, which, disaggregated, 
appear individually active, may be grouped so that the phzeno- 
mena of polarization, produced by their union, conceal or ren- 
der insensible the effects of their preper power, although it con- 
tinues to be exerted. These various cases proceed evidently 
from the conditions, purely molecular, supposed by our formule ; 
we must therefore be acquainted with their existence, and be 
able to recognise them, in order not to apply to them erroneously 
a theory adapted to quite another class of facts. 
31. For example; when a plate of quartz, regularly crystal- 
lized, is cut perpendicularly to the axis of the crystals, if a white 
luminous ray, previously polarized in a single direction, be 
passed through it normally, the planes of polarization of this 
ray, after its emergence, are deviated from their primitive di- 
rection, and unequally for the elements of unequal refrangibility. 
This fact was discovered by M. Arago in 1811*, On studying 
it a long time after, at various times, I found the following ex- 
_ perimental laws for it. In all the regular plates, cut from the 
* M. Arago published this fact in a memoir inserted among those of the 
class of the physical and mathematical sciences of the Institute of France, for 
1811, part 1. p. 93. This investigation is entitled “‘ Mémoire sur une modifica- 
tion remarquable qu éprouvent les rayons lumineux dans leur passage a@ travers 
certains corps diaphanes, ct sur quelques autres nouveaux phénomenes d’ optique.” 
My first researches on the rotatory phznomena observed in the plates of cry- 
stallized quartz, perpendicular to the axis, were two years later. They were 
published in the same collection for 1812, part 1. p. 218, but were only com- 
pleted in a later memoir, inserted in vol. ii. of the Collection of the Academy 
of Sciences, p. 41, 
