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constitution of the nucleus, and the integral variation of this charge from 1 to 92 gives the successive places of the periodic table. Expulsion of two Band one a-particle in any order gives an isotope of the original element with atomic weight four units less. Isobaric isotopes resulting in branch changes differ only in the internal structure and stability of the nucleus. The atomic mass is the only nuclear property known before the discovery of radioactivity, and, except as regards this, the whole of physics and chemistry up to the close of the nineteenth century had not penetrated beyond the outer electronic shell of the atom. Even now, mass and radioactivity remain the sole nuclear properties known.
Nemesis, swift and complete, has indeed overtaken the most conservative conception in the most conservative of sciences. The first phase robbed the chemical element of its time-honoured title to be considered the ultimate unchanging constituent of matter; but since its changes were spontaneous and beyond the power of science to imitate or influence to the slightest degree, the original conception of Boyle, the practical definition of the element as the limit to which the analysis of matter had been pushed, was left essentially almost unchanged.
The century that began with Dalton and ended with the discoveries of Becquerel and the Curies took the existing practical conception of the chemical element and theorised it almost out of recognition. The element was first atomised, and then the atom was made the central conception of the theory of the ultimate constitution of matter, on which modern chemistry has been reared, and from which its marvellous achievements, both practical and theoretical, have mainly sprung. The atom and the
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
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