Part 3

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paper on Alpine Structure is a reprint from "Radioactivity and Geology," which for the sake of completeness is here included. It is directed to the elucidation of a detail of mountain genesis: a detail which enters into recent theories of Alpine development. The weakness of the theory of the "horst" is manifest, however, in many of its other applications; if not, indeed, in all.

The foregoing essays on the physical influences affecting the surface features of the Earth are accompanied by one entitled The Abundance of Life. This originated amidst the overwhelming presentation of life which confronts us in the Swiss Alps. The subject is sufficiently inspiring. Can no fundamental reason be given for the urgency and aggressiveness of life? Vitality is an ever-extending phenomenon. It is plain that the great principles which have been enunciated in explanation of the origin of species do not really touch the problem. In the essay--which is an early one (1890)--the explanation of the whole great matter is sought--and as I believe found--in the attitude of the organism towards energy external to it; an attitude which results in its evasion of the retardative and dissipatory effects which prevail in lifeless dynamic systems of all kinds.

Other Minds than Ours? attempts a solution of the vexed question of the origin of the Martian "canals." The essay is an abridgment of two popular lectures on the subject. I had previously written an account of my views which carried the enquiry as far as it was in

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my power to go. This paper appeared in the "Transactions of the Royal Dublin Society, 1897." The theory put forward is a purely physical one, and, if justified, the view that intelligent beings exist in Mars derives no support from his visible surface features; but is, in fact, confronted with fresh difficulties.

Pleochroic Haloes is a popular exposition of an inconspicuous but very beautiful phenomenon of the rocks. Minute darkened spheres--a microscopic detail--appear everywhere in certain of the rock minerals. What are they? The discoveries of recent radioactive research--chiefly due to Rutherford--give the answer. The measurements applied to the little objects render the explanation beyond question. They turn out to be a quite extraordinary record of radioactive energy; a record accumulated since remote geological times, and assuring us, indirectly, of the stability of the chemical elements in general since the beginning of the world. This assurance is, without proof, often assumed in our views on the geological history of the Globe.

Skating is a discourse, with a recent addition supporting the original thesis. It is an illustration of a common experience--the explanation of an unimportant action involving principles the most influential considered as a part of Nature's resources.

The address on The Latent Image deals with 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

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.

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THE BIRTH-TIME OF THE WORLD

LONG ago Lucretius wrote: "For lack of power to solve the question troubles the mind with doubts, whether there was ever a birth-time of the world and whether likewise there is to be any end." "And if" (he says in answer) "there was no birth-time of earth and heaven and they have been from everlasting, why before the Theban war and the destruction of Troy have not other poets as well sung other themes? Whither have so many deeds of men so often passed away, why live they nowhere embodied in lasting records of fame? The truth methinks is that the sum has but a recent date, and the nature of the world is new and has but lately had its commencement."

Thus spake Lucretius nearly 2,000 years ago. Since then we have attained another standpoint and found very different limitations. To Lucretius the world commenced with man, and the answer he would give to his questions was in accord with his philosophy: he would date the birth-time of the world from the time when

A lecture delivered before the Royal Dublin Society, February 6th, 1914. Science Progress, vol. ix., p. 37

De Rerum Natura, translated by H. A. J. Munro (Cambridge, 1886).

poets first sang upon the earth. Modern Science has along with the theory that the Earth dated its beginning with the advent of man, swept utterly away this beautiful imagining. We can, indeed, find no beginning of the world. We trace back events and come to barriers which close our vista--barriers which, for all we know, may for ever close it. They stand like the gates of ivory and of horn; portals from which only dreams proceed; and Science cannot as yet say of this or that dream if it proceeds from the gate of horn or from that of ivory.

In short, of the Earth's origin we have no certain knowledge; nor can we assign any date to it. Possibly its formation was an event so gradual that the beginning was spread over immense periods. We can only trace the history back to certain events which may with considerable certainty be regarded as ushering in our geological era.

Notwithstanding our limitations, the date of the birth-time of our geological era is the most important date in Science. For in taking into our minds the spacious history of the universe, the world's age must play the part of time-unit upon which all our conceptions depend. If we date the geological history of the Earth by thousands of years, as did our forerunners, we must shape our ideas of planetary time accordingly; and the duration of our solar system, and of the heavens, becomes comparable with that of the dynasties of ancient nations. If by millions of years, the sun and stars are proportionately venerable. If by hundreds or thousands of millions of

years the human mind must consent to correspondingly vast epochs for the duration of material changes. The geological age plays the same part in our views of the duration of the universe as the Earth's orbital radius does in our views of the immensity of space. Lucretius knew nothing of our time-unit: his unit was the life of a man. So also he knew nothing of our space-unit, and he marvels that so small a body as the sun can shed so much, heat and light upon the Earth.

A study of the rocks shows us that the world was not always what it now is and long has been. We live in an epoch of denudation. The rains and frosts disintegrate the hills; and the rivers roll to the sea the finely divided particles into which they have been resolved; as well as the salts which have been leached from them. The sediments collect near the coasts of the continents; the dissolved matter mingles with the general ocean. The geologist has measured and mapped these deposits and traced them back into the past, layer by layer. He finds them ever the same; sandstones, slates, limestones, etc. But one thing is not the same. Life grows ever less diversified in character as the sediments are traced downwards. Mammals and birds, reptiles, amphibians, fishes, die out successively in the past; and barren sediments ultimately succeed, leaving the first beginnings of life undecipherable. Beneath these barren sediments lie rocks collectively differing in character from those above: mainly volcanic or poured out from fissures in