handed hemihedral crystals turned to the right and the
left-handed ones to the left the plane of polarization, and
when I took equal weights of each of these kinds of crys-
tals the mixed solution was neutral to polarized light
because of the neutralization of the two equal and oppo-
site individual deviations."

We can understand how in the presence of this un-
expected phenomenon, with its almost dazzling confirma-
tion of his preconceived idea, Pasteur received such a
shock that he quitted the laboratory, incapable of again
applying his eye to the polariscope. This was a clear
ray of sunlight coming to illuminate perspectives which
he had thus far examined only in shadow or half light.
Now that they were suddenly illuminated it was not the
time to abandon them.

The more so as immediately there was a harvest to be
reaped. In removing chemically from the right-handed
hemihedral crystals the tartaric acid which they con-
tained, he found an acid which, when compared minutely
with the acid of the grape, was found to be absolutely
identical with it. The left-handed crystals furnished
him furthermore a tartaric acid also identical in every
respect with the acid of the grape, save in one point,
that is that it bore on the left the hemihedral facet which
the first bore on the right, and that its solutions deviated
to the left exactly the same amount as equally concen-
trated solutions of the tartaric acid of the grape deviated
to the right. When these solutions were mixed there
was no deviation, and one obtained a third tartaric acid,
the paratartaric acid inactive by compensation. Fur-
thermore, this acid did not result from a juxtaposition
of these two constituents but from their combination,
for properly concentrated solutions of right-handed and
of left-handed tartaric acids often give off much heat
when mixed, and the liquid solidifies on the spot with an

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