r), but such a compound atom
conducts itself as if it were a simple and indivisible atom, as regards
the amount of space that separates it from its fellows under given
conditions of pressure and temperature. The compound atom, composed
of two or more elementary atoms, Avogadro proposed to distinguish, for
purposes of convenience, by the name molecule. It is to the molecule,
considered as the unit of physical structure, that Avogadro's law
applies.

This vastly important distinction between atoms and molecules, implied
in the law just expressed, was published in 1811. Four years later, the
famous French physicist Ampere outlined a similar theory, and utilized
the law in his mathematical calculations. And with that the law of
Avogadro dropped out of sight for a full generation. Little suspecting
that it was the very key to the inner mysteries of the atoms for which
they were seeking, the chemists of the time cast it aside, and let it
fade from the memory of their science.

This, however, was not strange, for of course the law of Avogadro is
based on the atomic theory, and in 1811 the atomic theory was itself
still being weighed in the balance. The law of multiple proportions
found general acceptance as an empirical fact; but many of the leading
lights of chemistry still looked askance at Dalton's explanation of this
law. Thus Wollaston, though from the first he inclined to acceptance of
the Daltonian view, cautiously suggested that it would be well to use
the non-committal word "equivalent" instead of "atom"; and Davy, for
a similar reason, in his book of 1812, speaks only of "proportions,"
binding himself to no theory as to what might be the nature of these
proportions.

At least two great chemists of the time, however, adopted the atomic
view with less reservation. One of these was Thomas Thomson, professor
at Edinburgh, who, in 1807, had given an outline of Dalton's theory in
a widely circulated book, which first brought the theory to the general
attention of the chemical world. The other and even more noted advocate
of the atomic theory was Johan Jakob Berzelius. This great Swedish
chemist at once set to work to put the atomic theory to such tests as
might be applied in the laboratory. He was an analyst of the utmost
skill, and for years he devoted himself to the determination of the
combining weights, "equivalents" or "proportions," of the different
elements. These determinations, in so far as they were accurately