atomic weight, with an equal number of electrons circulating about this
nucleus in rings. Bohr's theory, which is not very different from this,
has perhaps even more friends, and it is supported by the remarkable
discoveries of the lamented Moseley. But we must not take such theories
too seriously. As Kayser has said, any true theory of the make-up of the
atoms must assume an absolutely full and perfect knowledge of all
electrical and optical processes, and is therefore beyond our dreams. Or
as Professor Planck said in his Columbia lectures, we are not entitled
to hope that we shall ever be able to represent truly through any
physical formulæ the internal structure of the atom.

[Footnote 2: _Nature_, April 5, 1917.]


III

2. We must now take up the second phase of our subject, the problem of
the origin of matter.

Before we knew anything of radioactivity we could have dismissed such a
subject briefly by quoting the law of the conservation of matter, which
says that matter can neither be created nor destroyed by any means known
to science. By our knowledge of radioactivity we can make our answer a
little more learned, a little less abrupt, but none the less
discouraging to the advocate of the development hypothesis. We can tell
how the elements of high atomic weight, such as uranium and thorium, are
constantly giving off particles and are thus by loss or decomposition
being changed over into other elements, such as radium, niton, polonium
and lead. But our new knowledge compels us ultimately to give the same
answer as before, namely, that _we still do not know how matter ever