the normal school and able to enter the laboratory as
"préparateur," he made ready to pursue them. In
order to accustom the eye and the hand to the things
with which crystallography deals, he conceived the ex-
cellent idea of taking as guide a rather extended treatise
on crystalline forms, proposing to repeat all the experi-
ments and all the measurements, and to compare his
results with those of the author whom he followed step
by step. He chose for this purpose a work by Provostaye
on the tartrates, a most fortunate choice, for among the
substances endowed with rotary power, the tartrates
are those which present in simplest form the phenomena
toward which the ambition of the young savant directed
him. With other salts he would have been obliged to
search much longer to find things not so clear, but he
would have found them in the end.

He had, in fact, constantly present in his mind, this
correlation between hemihedrism and the rotary power
discovered in quartz. It was useless to say that it had
no apparent connection with the case of tartaric acid,
that is, that it resided in the arrangement of the mole-
cules, instead of in the molecule itself; the ideas of his
master as well as his own, reverting constantly to this
subject, told him that there ought to be something exter-
nal indicating the mode of arrangement of the atoms.
One of the best proofs that he searched for this some-
thing which his imagination had glimpsed in the memoirs
of Biot and Herschel, is that he saw at once on the
crystals of tartaric acid and the tartrates those hemi-
hedral facets which neither Provostaye nor Mitscherlich
had observed. The former, a conscientious worker but
without inspiration (sans flamme), had certainly seen
them but he had disregarded them. The second, whose
fame is well established, was occupied in his study espe-
cially with showing the isomorphism of the tartrates,

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