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product of polonium, actinium and other of the new radio-elements.

Gradually the tangled and complex succession of changes being undergone by uranium and thorium have been straightened out, and it is probable that the work is now complete. Some of the changes require millions of years, some are over in a billionth of a second or less. The atom of uranium expels 7 a- and 5 B-particles, in twelve successive changes, one particle per atom at each change. The atom of thorium expels 6 a-and 3 B-particles. The B-particles are atoms of electricity rather than of matter, and their expulsion affects the mass of the parent atom to only a negligible extent. But the a-particles are atoms of helium and the expulsion of each particle must lower the atomic mass of the parent atom by

4 units.

So long as the process of disintegration of the atom is proceeding, the rays emitted and the energy they possess afford the necessary evidence for their experimental study. But when it is all over, how are we to proceed? The final product into which uranium or thorium turns, if it is the final product, by hypothesis emits no rays. The quantity produced from any manageable quantity of uranium or thorium in a lifetime is too small to detect chemically. How can we find out even what it is?

There is the method that already had indicated helium as the element constituting the a-particle. In the natural radioactive minerals one would expect to find the end products of the radioactive changes in greater or less relative abundance, according as the mineral is geologically ancient or modern. This evidence for long indicated the element lead as the final product of the changes of uranium. To-day we know that the radioactive minerals are in reality geological clocks, and they record more accurately

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101 than in any other way the age of the stratum in which they occur. In a uránium mineral, for example, each i per cent. of lead in terms of the quantity of uranium signifies the lağze of a period of 80,000,000 years. Errors of course are possible, if lead should have been an original constituent of the mineral, but these are minimised by taking a large number of different minerals. On the other hadd every cubic centimetre by volume of helium per gram: of uranium in a uranium mineral signifies 9,000,000 years, and—as here helium, being a gas that forms no compounds, cannot have been initially present, and as, moreover, some will have escaped—the age of the mineral by this method is a minimum, whereas the age by the lead content may be too high. The carboniferous rocks tested by this new method appear to have an age of some 350,000,000 and the oldest Archean rocks of over 1,500,000,000 years.

The actual production of lead has not yet been proved directly in the same way as the production of helium has, though, but for the war, in all probability this would now have been accomplished. But even without the actual direct proof of this kind there is practically no room for doubt on the point. Indeed by a very important development, about which a few words may be said in conclusion, we know that not only uranium but also thorium both produce the element lead as the final product, and though the lead from uranium is absolutely identical chemically and spectroscopically with the lead from thorium, yet they are different. Stranger still, the lead which chemists are familiar with as one of the elements is probably a mixture of both kinds.

We have seen that the expulsion of an a-particle · ought to lower the atomic weight of the element expelling it by 4 units, 4 being the atomic weight of helium. In its transformation into radium, uranium


expels 3 a-particles.: Ihe atomic weight of uranium is 238, and that found by Mme. Curie for radium is 226. So far: so good. Radium in its further changes expels. '5' a-particles, and the atomic weight of the end product should be therefore 206. The atomic weight of thorium is 232, and, as it expels 6. particles in all, that of the end product of thörðum should be 208. The atomic weight of Ordinary lead is 207•2. The atomic weight of bismuth is 208, but the writer was unable to find in a special examination of over 20 kilograms of a certain thorium mineral even a trace of bismuth, though there was 0:3 per cent. of lead. This definitely rules bismuth out.

In the early months of 1913 a fundamental step forward was taken into our knowledge of the nature of matter which started from the discovery of the simple complete law of elementary evolution as we have come to know it in radioactive change, which is largely due to two of the writer's old students, A. S. Russell and A. Fleck. The expulsion of the a-particle, or the B-particle, from an atom leaves a new atom with properties different from the parent, but different in a very definite and striking way. If the particle expelled is the a-particle, the element after this expulsion invariably changes its whole chemical character and passes from the place it occupies in the Periodic Table to a new place, next but one to it in the direction of diminishing atomic weight. If the expelled particle is a B-particle the change of place is invariably into the next place in the opposite direction. After three changes in any order, one a- and two ß-,-a very common sequence in the series,—the element returns to the place it first occupied. Its atomic weight is less than it was by 4 units, but in its whole chemical nature and even in its spectrum, it is not merely like its original

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parent. It is chemically identical with it. Elements which so occupy the same place in the Periodic Table and are absolutely identical in all their chemical properties are called isotopes. The recognition of such isotopes is fundamentally new, and cuts more deeply into old-established ideas of the nature of matter than even the surprising discoveries of the genesis of one element out of another.

The present theory of atomic structure is due to Rutherford, and is based on experiments on the course followed by an a-particle when it ploughs its way through the atoms of matter. These experiments have shown that the atom consists of a central nucleus, possessing all but a negligible part of the atomic mass but occupying only an exceedingly ' minute fraction of the atomic volume. The nucleus contains a preponderance of positive charges and is surrounded by an equivalent number of separate negative electrons, revolving in a system around it. This theory lent itself at once to the interpretation of the new developments here referred to, and both together, along with very important work by the late H. G. J. Moseley on the wave-lengths of the X-ray spectra of the elements, have furnished the key to the deciphering of the Periodic Law. It is melancholy to record that Moseley fell at Suvla Bay, aged only twenty-eight.

Prior knowledge of the atoms of matter has been superficial in the literal sense-confined entirely to the outermost shell of the atom. We have now penetrated to the interior and find, first, an inner shell, wherein X-rays take their origin, and, secondly, still further to the nucleus, the sanctum sanctorum of the atom, revealed only by radioactivity and alone concerned in this phenomenon. The same outer and inner shells—that is, the same kind of atom to the older knowledge-may contain demonstrably different nuclei. Matter is of indefinitely more kinds than the chemist and his Periodic Law have disclosed.

The places in the Periodic Table represent integral nett charges of electricity in the constitution of the nucleus. The expulsion of the a-particle with its double charge of positive electricity shifts the element in the Periodic Table by two places in one direction and the expulsion of the B-particle, with its single charge of negative electricity, shifts it one place in the other direction. Nature does not deal in fractions of an atom of electricity any more than with fractions of an atom of matter. As we pass from hydrogen, at the beginning, to uranium, at the end, of the elements, we pass 92 places in the Periodic Table, each element differing from the one preceding it by a unit charge or "atom" of positive electricity in its nucleus, Hydrogen has one such and uranium 92 such unit positive charges. The number expressing the element's place in the Periodic Table is called the atomic number. It is the nett number of charges in the atomic nucleus, i.e. the difference between the separate positive and negative charges. Before the discovery of the radio-elements the following represented the last 14 places of the Periodic Table :

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