and left-handed plagihedrons oblige us to admit as
present in the quartz, we can place the parallel, but
inverse, actions which it has on polarized light.

I have just spoken of structure. It is necessary
to make here an important remark: this action on
polarized light manifests itself only in crystallized quartz.
With the amorphous quartz, or silicia in solution
in any liquid whatsoever, we no longer find a trace of
it. Furthermore, the action takes place only on a ray
of polarized light traversing the crystal in the direction of
its longer axis and parallel to that axis, or at least in a
direction very little inclined away from it. It dimin-
ishes rapidly in proportion to the augmentation of the
inclination, and there is no longer a trace of it when
the ray traverses the crystal obliquely and in the direc-
tion of its shorter diameter.

This circumstance, which connected the rotary power
with the molecular files of Delafosse, was so much
the more curious as it did not occur at all in the other
substances in which Biot had also discovered the rotary
power. Almost all of these substances were products of
animal or vegetable life: sugar, tartaric acid, different
essences, albumen, etc. But those which could crystal-
lize, the sugar and the tartaric acid, had no polarizing
action in the crystalline state. All, on the contrary,
when dissolved in water or any liquid whatsoever, rotated
the plane of polarization, some to the right, some to the
left. This rotation is always the same for the same solu-
tion when the density is the same, regardless of the direc-
tion in which the light ray is made to traverse the liquid
which is being examined, and we can agitate this liquid
during the observation without changing in any way the
quantity and direction of the rotation, a fact which well
demonstrates that it does not depend on the internal
arrangement of the active molecules in the solvent.

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