etermining the atomic weights
would be very simple. We should merely have to take some one convenient
element as a standard, and find by experiment how much of each other
element would combine with a fixed weight of it. The ratios thus found
would be the same ratios as those between the atoms of the elements, and
thus we should have their relative atomic weights. The law of multiple
proportion calls attention to the fact that the atoms combine in other
ratios than 1: 1, and there is no direct way of telling which one, if
any, of the several compounds in a given case is the one consisting of a
single atom of each element.

If some way were to be found of telling how much heavier the entire
molecule of a compound is than the atom chosen as a standard,--that is,
of determining the molecular weights of compounds,--the problem could be
solved, though its solution would not be an entirely simple matter.
There are ways of determining the molecular weights of compounds, and
there are other experiments which throw light directly upon the relative
weights of the atoms. These methods cannot be described until the facts
upon which they rest have been studied. It will be sufficient for the
present to assume that these methods are trustworthy.

~Standard for atomic weights.~ Since the atomic weights are merely
relative to some one element chosen as a standard, it is evident that
any one of the elements may serve as this standard and that any
convenient value may be assigned to its atom. At one time oxygen was
taken as this standard, with the value 100, and the atomic weights of
the other elements were expressed in terms of this standard. It would
seem more rational to take the element of smallest atomic weight as the
standard and give it unit value; accordingly hydrogen was taken as the
standard with an atomic weight of 1. Very recently, however, this unit
has been replaced by oxygen, with an atomic weight of 16.

~Why oxygen is chosen as the standard for atomic weights.~ In the
determination of the atomic weight of an element it is necessary to find
the weight of the element which combines with a definite weight of
another element, preferably the element chosen as the standard. Since
oxygen combines with the elements far more readily than does hydrogen to
form definite compounds, it is far better adapted for the standard
element, and has accordingly replaced hydrogen as the standard. Any
definite value might be given to the weight