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In the second place a serious attempt has been made to use both
of the two main competing theories, usually known as the
molecular-orbital and valence-bond methods. Both are approximations,
whose range of validity is now sufficiently understood for us to
recognize the folly of trusting to either alone. Earlier accounts
have tended to emphasize one of these to the exclusion of the
other. This will no longer do : and so both have been developed
in this book. At first they are treated separately, hut in the final
third part they are used almost indiscriminately. The subject
matter of valence has been, and still is, changing rapidly, as befits
a living discipline; but there is now much that seems to have
'settled down'; and we may reasonably claim that most of the
material in the first eleven chapters would oommand universal
agreement. The twelfth and last chapter differs from the rest in
that it indicates the present position in several matters where
opinion is by no means unanimous, and where further progress is
to be expected even now.
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