The answer to this question (Is there any satisfactory
evidence deducible of the existence of two distinct forms of chemical
combination?) depends materially on the view we take of the property
called in text-books valency or atomicity; and before discussing the
question it is important to have a clear idea of what these words
valency and atomicity really mean. It is necessary, too, to start with
some propositions which must be taken for granted. These propositions
are: First, that in all chemical changes, those kinds of matter which we
commonly call elementary, do not suffer decomposition. Second, That the
atomic weights of the elements as received are correct, i.e., that they
do really express with great exactitude the relative weights of the
atoms of the individual elements. If we accept these two propositions,
it follows that hydrogen can be replaced atom for atom by other elements
not only by the hydrogens but by alkali metals, etc. Hydrogen is, it may
here be remarked, an element of unique character; not only can it be
replaced by the elements of the widely different classes represented by
chlorine and sodium, but it is the terminal of the series of paraffins,
C_{n}H_{2n}; C_{3}H_{6}, C_{2}H_{4}, H_{2}.
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