Some of the closely related discoveries, such as the fact that the
X-rays show a spectrum susceptible of examination, were not so
disconcerting in themselves; but the marvellous pictures of the
structure of the atom elicited by these discoveries made many good
people almost question whether our venerable experimenters had not been
indulging in pipe dreams amid their laboratory work.

Do we, then, begin to understand the real composition of matter? Does it
have component parts, in the materialistic sense; or is what we call
_matter_ only a mysterious manifestation of energy? And if the latter
be our answer, can we hope to settle the problem objectively and so
conclusively that it will stay settled? In short, do we, regarding these
border-line subjects between metaphysics and natural science, know
anything more than our fathers and our grandfathers?

It will be convenient to consider these problems under two heads: the
composition of matter, and the origin of matter.


II

1. It was long ago recognized that matter must be composed of particles
which are driven farther apart by heat and are brought closer together
by cold, thus laying the foundation for the theory of the molecular
composition of matter. But not until the time of Dalton, about a hundred
years ago, was it proved that the molecule itself, the unit of physical
change, is capable of definite division into atoms, the units of
chemical change. This conception of the molecules and atoms as the
ultimate units of which matter is composed maintained its place until
the discovery of radioactivity and its associated phenomena, about