sometimes finds in the cavities between the large crystals
of this acid some little needle-like crystal forming
tufts which are visible as an opaque white mass on the
surface of the semitransparent tartaric acid and some-
times so much resemble oxalic acid crystals that in the
factory of Thann, where they were formerly very abun-
dant, they have been taken for oxalic acid crystals and
an attempt made to sell them as such. It was soon
recognized that they were formed of a particular kind
of tartaric acid, giving salts entirely similar to the tar-
trates. Mitscherlich who made a comparative study
of the known tartrates and of these new salts,
which he called paratartrates, found them identical
in all their relations. They had the same crystal-
line form, the same specific gravity, the same double
refraction in the crystalline state, the same angle
of the optical axes, the same index of refraction when
they were dissolved in the same proportion of water.
In short, no method, either physical or chemical, made it
possible to distinguish them, and they seemed identical
in every respect, save this, that the tartrates acted on
polarized light while the paratartrates were entirely
without action.

Having arrived at this stage in his researches, Pasteur
could not fail to be impressed by this apparent contradic-
tion. " Mitscherlich was deceived," he said, "in affirm-
ing that the crystals of the tartrates and the paratartrates
resemble each other. There must be some external differ-
ences between them as regards the hemihedral facets.
Mitscherlich, preoccupied with his ideas on isomorphism
which made much of all the crystalline resemblances
between the different forms, would not have seen these
differences which he did not seek, but I, who have the
preconceived idea of their existence, am in a good posi-
tion to find them if they are there."

-17-