identical spectra. For this element, lead, Rutherford and Andrade have shown that the secondary y-radiation excited by the impact of B-rays on a block of ordinary lead gave by crystal reflection two lines identical in wave-length with the two strongest lines in the y-ray spectrum of radium-B, an isotope of lead, as Fleck showed, of atomic weight 214. This is of importance as indicating that X-rays and y-rays, although no doubt originating in a deeper region of the atom than the ordinary light spectrum, do not originate in the deepest region of all to which the weight of an atom and its radioactive properties are to be referred.

DESCRIPTION OF THE FIGure.

The generalisation, brought up to date, is set forth in detail in the Tables on p. 134 and is illustrated by the accompanying figure, which is to be read at an angle of 45°, making the lines of atomic weight horizontal and the division between the successive places in the periodic table vertical. Starting from uranium and thorium, the series run in an alternating course across the table and extend over the last twelve places as far as the element thallium. At this point, it is interesting to note that the expulsion of an a- instead of a B-particle would have resulted in the production of an isotope of gold, and so literally have realised the goal of the alchemist. As it happens, a B-particle is expelled and lead results, so far as the changes have yet been traced, in all cases as the final product.

It has been necessary, in order to separate the series from one another, to displace the actinium series to the right and the radium series to the left of the centre of the places, but this displacement within the single place is not intended to express

DESCRIPTION OF THE FIGURE

* 133

any physical significance; but for the fact that many members would be superimposed, they would all be represented in the centre of the places. The periods of average life, which are always 1.443 times the periods of half-change, are shown for each member above or below its symbol, a ? indicating that the period is estimated indirectly from the Geiger-Nuttall relation.

The figures at the head of each place represent the atomic numbers or number of the place in the periodic table, starting with hydrogen as unity, helium as 2, lithium as 3, and so on. Moseley found that the square-root of the frequency of the characteristic X-radiation of an element was, for the K-series of radiations, proportional to integers less by one than the atomic numbers. Strictly speaking, there is no means of determining the absolute value of the atomic number, but the starting point having been fixed for any one element, the others can then be found in terms of it. Moseley assumed the atomic number of aluminium as 13, as it is the thirteenth known element in the list starting with hydrogen as unity. It is unlikely that any new elements will be discovered between hydrogen and aluminium, although if they were it would be necessary to alter the whole of the subsequent atomic numbers to correspond. For X-radiations of the other series, the square-roots of the frequencies are not proportional to integers even, although the differences are nearly integral for successive elements in the periodic table. The actual numbers in the figure, 92 for uranium, for example, are derived from the assumption that the atomic number of aluminium is 13, but it is well to remember that, although relatively to one another based on experimental evidence, the absolute value is to some extent arbitrary.

T

Sequence of Changes of Uranium (U) and Thorium (Th) into various

Isotopes of Lead (Pb.)

[Face page 134.