important this matter of architecture of the molecule--of space
relations of the atoms--may be was demonstrated as long ago as
1823, when Liebig and Wohler proved, to the utter bewilderment of
the chemical world, that two substances may have precisely the
same chemical constitution--the same number and kind of
atoms--and yet differ utterly in physical properties. The word
isomerism was coined by Berzelius to express this anomalous
condition of things, which seemed to negative the most
fundamental truths of chemistry. Naming the condition by no
means explained it, but the fact was made clear that something
besides the mere number and kind of atoms is important in the
architecture of a molecule. It became certain that atoms are not
thrown together haphazard to build a molecule, any more than
bricks are thrown together at random to form a house.

How delicate may be the gradations of architectural design in
building a molecule was well illustrated about 1850, when Pasteur
discovered that some carbon compounds--as certain sugars--can
only be distinguished from one another, when in solution, by the
fact of their twisting or polarizing a ray of light to the left
or to the right, respectively. But no inkling of an explanation
of these strange variations of molecular structure came until the
discovery of the law of valency. Then much of the mystery was
cleared away; for it was plain that since each atom in a molecule
can hold to itself only a fixed number of other atoms, complex
molecules must have their atoms linked in definite chains or
groups. And it is equally plain that where the atoms are
numerous, the exact plan of grouping may sometimes be susceptible
of change without doing violence to the law of valency. It is in
such cases that isomerism is observed to occur.