is he to see justly, that is to say, to judge with the judg-
ment of that time? How abstract oneself from what has
been learned since, and take on again the necessary
ignorance?

Nevertheless, I shall attempt to do this throughout
this volume; but, as it is easy to understand, the greatest
difficulties are at the beginning. Familiar as we are
to-day with the theories of molecular structure, we have
some difficulty in picturing to ourselves the chaotic
condition of these ideas among the scientific men of 1840.

They had a knowledge of the chemical molecule.
They knew it is formed by a grouping of generally quite
stable atoms, the number, weight and nature of which
are ordinarily very well defined. They knew, for ex-
ample, that there is one atom of chlorine and one of
sodium in marine salt, while in calcium carbonate there
is one atom of calcium, one atom of carbon, and three
atoms of oxygen. They had recognized that the different
compound molecules differentiated themselves ordinarily
by the number and nature of the atoms composing them;
that there are, nevertheless, some which contain the same
number of the same atoms without being identical, from
which one was led to suppose that they were arranged
somewhat differently. But in what did these arrange-
ments consist? How do the atoms dispose themselves
in relation to each other in a molecule? What is the
resultant form for this molecule? These were questions
on which no one had clear ideas.

Crystallography had given no answer, contrary to
what we might believe to-day, after the teachings which
this science has furnished us It held to Haüy's narrow
and geometrical conception of the integral crystal mole-
cule. We know that he called by this name the little
solid, the juxtaposition and superposition of which in an
infinite number resulted in the formation of the crystal.

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