Phil. Mag. (1914), P. 703.

The first part of this paper dealt with a method of photographing X-ray spectra, and included the spectra of a dozen elements. More than thirty other elements have now been investigated, and simple laws have been found which govern the results, and make it possible to predict with confidence the position of the principal lines in the spectrum of any element from aluminium to gold. The present contribution is a general preliminary survey, which claims neither to be complete nor very accurate....

The results obtained for radiations belonging to Barkla's K series are given in Table I, and for convenience the figures already given in Part I. are included. The wave-length X has

been calculated from the glancing angle of reflexion @ by means of the relation nλ = 2 sin 0, where d has been taken to be 8.454 × 10-8 cm. As before, the strongest line is called a and the next line B. The square root of the frequency of each line is plotted in Fig. 3, and the wave-lengths can be read off with the help of the scale at the top of the diagram.

The spectrum of Al was photographed in the first order only. The very light elements give several other fainter lines, which have not yet been fully investigated, while the results for Mg and Na are quite complicated, and apparently depart from the simple relations which connect the spectra of the other elements.

In the spectra from yttrium onwards only the a line has so far been measured, and further results in these directions will be given in a later paper. The spectra both of K and of Cl were obtained by means of a target of KCl, but it is very improbable that the observed lines have been attributed to the wrong elements. The a line for elements from Y onwards appeared to consist of a very close doublet, an effect previously observed by Bragg in the case of rhodium.

The results obtained for the spectra of the L series are given in Table II and plotted in Fig. 3. These spectra contain five lines, a, ß, y, d, e, reckoned in order of decreasing wave-length and decreasing intensity. There is also always a faint companion a' on the long wave-length side of a, a rather faint line between Bandy for the rare earth elements at least, and a number of very faint lines of wave-length greater than a. Of these, a, B, 6, and y have been systematically measured with the object of finding out how the spectrum alters from one element to another. The fact that often values are not given for all these lines merely indicates the incompleteness of the work. The spectra, so far as they have been examined, are so entirely similar that without doubt a, B, and y at least always exist. Often y was not included in the limited range of wave-lengths which can be photographed on one plate. Sometimes lines have not been measured, either on account of faintness or of the confusing proximity of lines due to impurities....

Conclusions.

In Fig. 3 the spectra of the elements are arranged on horizontal lines spaced at equal distances. The order chosen for the elements is the order of the atomic weights, except in the cases of A, Co, and Te, where this clashes with the order of the chemical properties. Vacant lines have been left for an element between Mo and Ru, an element between Nd and Sa, and an element between W and Os, none of which are yet known, while Tm, which Welsbach has separated into two constituents, is given two lines. This is equivalent to assigning to successive elements a series of successive characteristic integers. On this principle the integer N for Al, the thirteenth element, has been taken to be 13, and the values of N then assumed by the other elements

are given on the left-hand side of Fig. 3. This proceeding is justified by the fact that it introduces perfect regularity into the X-ray spectra. Examination of Fig. 3 shows that the values of for all the lines examined both in the K and the L series now fall on regular curves which approximate to straight lines. The same thing is shown more clearly by comparing the values of N in Table I with those of

v being the frequency of the line and v。 the fundamental Rydberg frequency. It is here plain that QK = N - 1 very approximately, except for the radiations of very short wave-length which gradually diverge from this relation. Again, in Table II a comparison of N with

where v is the frequency of the La line, shows that QL = N-7.4 approximately, although a systematic deviation clearly shows that the relation is not accurately linear in this case.

Now if either the elements were not characterized by these integers, or any mistake had been made in the order chosen or in the number of places left for unknown elements, these regularities would at once disappear. We can therefore conclude from the evidence of the X-ray spectra alone, without using any theory of atomic structure, that these integers are really characteristic of the elements. Further, as it is improbable that two different stable elements should have the same integer, three, and only three, more elements are likely to exist between Al and Au. As the X-ray spectra of these elements can be confidently predicted, they should not be difficult to find. The examination of keltium would be of exceptional interest, as no place has been assigned to this element.

Now Rutherford has proved that the most important constituent of an atom is its central positively charged nucleus, and van den Broek has put forward the view that the charge carried by this nucleus is in all cases an integral multiple of the charge on the hydrogen nucleus. There is every reason to suppose that