h a subject which had
been approached by various writers before the time of my essay;
but, so far as I know, an explanation

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based on the facts of photo-electricity had not been attempted.
Students of this subject will notice that the views expressed are
similar to those subsequently put forward by Lenard and Saeland
in explanation of phosphorescence. The whole matter is of more
practical importance than appears at first sight, for the
photoelectric nature of the effects involved in the radiative
treatment of many cruel diseases seems to be beyond doubt.

It was in connection with photo-electric science that I was led
to take an interest in the application of radioactivity in
medicine. The lecture on _The Use of Radium in Medicine_ deals with
this subject. Towards the conclusion of this essay reference will
be found to a practical outcome of such studies which, by
improving on the methods, and facilitating the application, of
radioactive treatment, has, in the hands of skilled medical men,
already resulted in the alleviation of suffering.

Leaving out much which might well appear in a prefatory notice, a
word should yet be added respecting the illustrations of scenery.
They are a small selection from a considerable number of
photographs taken during my summer wanderings in the Alps in
company with Henry H. Dixon. An exception is Plate X, which is by
the late Dr. Edward Stapleton. From what has been said above, it
will be gathered that these illustrations are fitly included
among pages which owe so much to Alpine inspiration. They
illustrate the

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subjects dealt with, and, it is to be hoped, they will in some
cases recall to the reader scenes which have in past times
influenced his thoughts in the same manner; scenes which in their
endless perspective seem to reduce to their proper insignificance
the lesser things of life.

My thanks are due to Mr. John Murray for kindly consenting to the
reissue of the essay on _The Birth-time of the World_ from the
pages of _Science Progress_; to Messrs. Constable & Co. for leave
to reprint _Pleochroic Haloes_ from _Bedrock_, and also to make some
extracts from _Radioactivity and Geology_; and to the Council of
the Royal Dublin Society for permission to republish certain
papers from the Proceedings of the Society.

_Iveagh Geological Laboratory, Trinity College, Dublin._

July, 1915.

xvi

THE BIRTH-TIME OF THE WORLD [1]

LONG ago Lucretius wrote: "For lack