The first ten chapters of this book have been mainly
descriptive. They have portrayed the material universe as seen by
an observer who has the ability to adjust his field of view to encompass
entire galaxies or single atoms. At the lowest level we have seen
how electrons can be arranged around nuclei in atoms, and how this
arrangement limits the different kinds of atoms that can exist.
At a slightly higher level of organization we have seen the way
in which electrons hold groups of atoms together in molecules of
definite size and shape, and how the properties of matter depend
on molecular structure. This is the essence of chemistry: to explain
matter in molecular terms.
Like any other branch of science, chemistry eventually becomes trivial
if it remains descriptive. The essence of science is control of
matter by means of successful predictions of behavior; and predictions
without measurement are hazy. Sooner or later we must adopt the
viewpoint of William Thomson (Lord Kelvin), a pioneer in thermodynamics
and electricity, who said in 1891:
"When you can measure what you are speaking about, and
express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meagre and unsatisfactory kind. It may be the beginning
of knowledge, but you have scarcely, in your thoughts, advanced
to the stage of science."