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of the variety of their physiognomy there are some
features which remain immutable among them and con-
stitute their family mark. These features are three
facets which always succeed each other in the same
order and make between them very nearly the same
angles. These facets, which consist in the primitive
form of two contiguous faces, P and M
( Fig. 3 ), and a facet b1, cutting off the
intersection of the first two, are parallel
to the same straight line which might re-
place for us that axis of the hexagonal
prism of quartz, which has been so useful
to us in establishing the correlation be-
tween the direction of the hemihedrism
and that of the rotary power in this
crystal.


FIG. 3. --Primitive
tartrate crystal.

Let us agree to place this right line vertically in our
tartrate crystals and to turn forward the group of three
facets which is that characteristic the different crystals
have in common. All the crystals can thus be
ranged, in spite of the variety of their forms, in an
oriented series like soldiers exhibiting in front the
same series of buttons. But when one has arranged
them thus he preceives with surprise that all of these
soldiers bear only one epaulet, turned in every case
in the same direction: I mean to say that all these tar-
trates have their hemihedral facet inclined forward to
the right of the observer.

If one turns them half-way around they are like
children's lead soldiers, or like the god Janus, inasmuch
as the front cannot be distinguished from the back:
the hemihedral facet from the rear is now in front, but
it is always to the right. If one reverses them in order
to observe them from the other end they resemble
then the double figures on playing cards; their extrem-

-15-

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Publication Information: Book Title: Pasteur: The History of a Mind. Contributors: Ėmile Duclaux - author, Erwin F. Smith - transltr, Florence Hedges - transltr. Publisher: W.B. Saunders Company. Place of Publication: Philadelphia. Publication Year: 1920. Page Number: 15.
    
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