element became synonyms, related as the singular to the plural, and implicit throughout this century was the assumption that all the atoms of any one element are identical with one another in every respect. The only exception is in Sir William Crookes's conception of "meta-elements" as applied to the rare earths. Here the idea was rather that of a gradual and continuous difference among the different atoms of the same element, the properties of the latter being the mean of those of its individual atoms. Modern developments have tended definitely away from rather than towards this view.

The second phase in the development of radioactive change has now negatived each and every one of the conceptions of last century that associated the chemical element with the atom. The atoms of the same chemical element are only chemically alike. Unique chemical and spectroscopic character is the criterion, not of a single kind of atom, but rather of a single type of external atomic shell. Different chemical elements may have the same atomic mass, the same chemical element may have different atomic masses, and, most upsetting of all, the atoms of the same element may be of the same mass and yet be an unresolvable mixture of fundamentally distinct things. Present-day identity may conceal differences for the future of paramount importance when transmutation is practically realised. Then it may be found that the same element, homogeneous in every other respect, may change in definite proportion into two elements as different as lead and gold. The goal that inspires the search for the homogeneous constituents of matter is now known to be, like infinity, approachable rather than attainable. The word homogeneity can in future only be applied, qualified by reference to the experimental methods available for testing it.

All this, of course, does not in the least affect or minimise the practical importance of the conception of the chemical elements as understood before these discoveries. Every chemist knows the conception has had and will continue to have a real significance as representing the limit of the spectroscopic and chemical analysis of matter which remains, although it now is known to convey something very different from the original and natural conception of the chemical elements as the Im n's of the material alphabet.

MATTER, ENERGY, CONSCIOUSNESS

AND SPIRIT1

THE feeling is gradually awakening in the consciousness of the community, that the discoveries and advances made by science in the past century are not such as they have been accustomed to be represented by people to whom they are a sealed book, as important to money-making and trade, for waging war and overtaking the heavy drudgery of the world, but in an altogether different category from humane studies. The scientific materialist in seeking to understand the external physical universe, and the relation in which men stand thereto, has invaded territories which formerly the humanist and theologian had to themselves, and made discoveries which are essential to the understanding of modern life and its problems. If it were necessary to make choice between the old and the new in its relation to the world of to-day, rather than in relation to some remote childhood of the world, the knowledge gained in the last hundred years surely is the part of the whole of knowledge which could least be spared. It is just this part which men who have to govern modern peoples, administer the affairs of present-day empires, and instruct and educate the youth of the world, usually know least about. That science has something to say apart from its

1 Lecture to the Aberdeen University Christian Union, Marischal College, 25th April 1919.

application to the material and utilitarian interests of men, that its revelation is both clear and inspiring, "a source not merely of material convenience but of spiritual elevation," as Mr Arthur Balfour has said, is, however, now being more generally understood.

Science has wrecked beyond repair certain dogmas and beliefs generally current prior to the development of the doctrine of evolution on the biological side. That doctrine has completely reversed the traditional outlook of men and turned their highest interest from the contemplation of the past to the problems of the future. But physical science, the science, in the first instance, of the inanimate world, contemporaneously with these great developments of biology, has contributed in its doctrine of energy an advance of direct and living human interest certainly not less, and possibly even of greater fundamental importance than the conception of evolution. It, therefore, is almost a duty of the scientific man, however little he may desire or feel himself competent for the task, to attempt to rebuild as well as destroy, and to state, so far as he can, what is his view of the matters in which hitherto the priest and the philosopher have, with insufficient knowledge of external nature, been left to themselves. Such a synthesis has been hitherto attempted, if at all, from the standpoint of biological science, with which, I need scarcely say, I am totally unfitted to deal. In approaching it from the purely physical standpoint, one has the very great advantage that one starts from a basis which now may be considered beyond controversy or cavil, and which even the phenomena of life cannot complicate or make obscure. On the other hand, the corresponding disadvantage is that one starts farther off from and has a greater distance to go

to reach the domain that has to be brought into reconciliation with external nature. The biologist dealing with life from the scientific standpoint has the more central position. The ultimate problems of matter and energy, on the one hand, and consciousness and spirit, on the other, lie equally outside his true domain, and are apt to appear, perhaps, equally inaccessible and mysterious. The physicist from his more extreme standpoint, completely outside of the realm of life, may not be able to see very far, but what he can see is seen with all the certainty and definiteness that distinguish and characterise the explanation of the phenomena with which he deals. Do not draw the hasty conclusion that, because the clarity and unanimity reached in the study of inanimate nature have not been approached in the study of life, they have therefore no application whatever to the higher aspects of life. On the contrary, I hope to show that, as regards what it is impossible to believe at least, they effect a not inconsiderable simplification, and so pave the way at least for a more definite and truer human philosophy to replace the old.

IMMORTALITY OR THE CONSERVATION OF
PERSONALITY.

Life, so far as our direct experience is concerned, is lived in an intimate relation with the external physical universe, and the breaking of that connection is death. Almost before men could count or reason correctly about the simplest phenomena, they have contended that life transcends the breaking of the bond between it and the external world and persists after it has departed from this world. The attitude of mind is very familiar in science, as in other fields. Amid a world of appearance and