§ 76. The Determination of the Atomic Weights of the
Elements.
With the acceptance of Dalton's Atomic Theory, it became necessary to
determine the atomic weights of the various elements, i.e., not
the absolute atomic weights, but the relative weights of the various
atoms with reference to one of them as unit.4
We cannot in this place enter upon a
discussion of the various difficulties, both of an experimental
and theoretical nature, which were involved in
this problem, save to remark that the correct atomic
weights could be arrived at only with the acceptance
of Avogadro's Hypothesis. This hypothesis, which is
to the effect that equal volumes of different gases
measured at the same temperature and pressure
contain an equal number of gaseous molecules, was
put forward in explanation of a number of facts
connected with the physical behaviour of gases; but
its importance was for some time unrecognised, owing
to the fact that the distinction between atoms and
molecules was not yet clearly drawn. A list of those
chemical substances at present recognised as "elements,"
together with their atomic weights, will be
found on pp. 106, 107.