VIII. On the Atomic Weight of Radium. By W. Marshall Watts, D.Sc.,F.I.C. [Communicated by the Author.] In a short paper read before the Physical Society [citation redacted] I have recently called attention to some relationships between the spectra of certain allied elements, and the squares of their atomic weights, which do not seem to be generally known. There appear to be two distinct kinds of connexion between the spectra of allied elements. In one class of cases, of which the family of zinc, cadmium, and mercury, and that of gallium and indium, furnish the best examples, the differences between the oscillation-frequencies of certain lines of the one element are to the differences between the oscillation-frequencies of the corresponding lines of the other element as the squares of their atomic weights : so that, if it be admitted that the lines do correspond, it is possible to calculate the atomic weight of the one element from that of the other by means of the spectra. In the other class of cases, of which the families potassium, rubidium, cesium, and calcium, strontium, and barium offer the best examples, the element of greater atomic weight has the smaller oscillation-frequency, and three elements are so related that the differences of oscillation-frequency between the elements, in comparing corresponding lines in their spectra, are proportional to the differences between the squares of the atomic weights ; so that we can calculate the atomic weight of one element from the atomic weights of two other elements of the same family by means of their spectra. These relationships are most easily seen by plotting squares of atomic weights as ordinates, and oscillation-frequencies as abscissas : it is then seen that — in the first case — the straight lines joining corresponding points in the homologous spectra intersect on the line of zero atomic weight ; and, in the second case, that the corresponding points in the three spectra compared lie on straight lines. It is further observed that these straight lines are very nearly parallel, although they seem to converge slightly. As an example, the following lines in the arc-spectra of Barium Strontium Calcium [table redacted] [header] 65 Taking the atomic weights of calcium and strontium on the oxygen scale as 40" 1 and 87*6, we may then calculate the atomic weight of barium on the assumption that the points assumed to correspond in the three spectra have some physical connexion, and that they do actually lie upon straight lines. We thus get by calculation for the atomic weight of barium [table redacted] It would seem that similar relationships hold, to a certain extent, between the spectrum of radium and the spectra of mercury, barium, and calcium, and in the present communication I have made an attempt to calculate the atomic weight of radium from these relationships. They seem to me too remarkable to be attributed to chance. Both kinds of relationship seem to exist. The brightest lines in the spectrum of radium are 20714*8 of intensity (10), 21350-9 (14;, and 26207*7 (16). There are two lines in the barium spectrum, 35892*6 (8r), and 36070*2 (6), which seem to correspond to the first two radium lines, and to a calcium line at 43930*4 (8r). There does not appear to be a second calcium line at this place, but it is situated in the extreme violet, and it is quite possible that a second line may have escaped observation. Taking the calcium line to correspond to the less refrangible barium and radium lines, by calculating from the atomic weights Ba= 137*4 and Ca = 40*l, we get 2'2fr3'2 for the atomic weight of radium. The radium line 26207*7 lies on a straight line with mercury 31982*2 (10r) f and barium 42807*0 (8r). Calculating from Hg = 200 and Ba = 137*4 we get 226*42 for the atomic weight of radium. Other correspondences of the like kind seem to be Radium. Mercury. Barium. Calcium. [table redacted] The connecting lines in these five cases are very nearly [footer] 66 [header] parallel to the five lines for barium, strontium, and calcium given as examples, but the whole ten lines exhibit a slight convergence towards the same point. Taking now the other mode of correspondence mentioned first, which is illustrated by the homology of the triplets (and many other lines) in the spectra of zinc and cadmium, we find the following connexion between the spectra of radium, mercury, and barium. Lines meeting on the Line of Zero Atomic Weight at Oscillation-Frequency of about 44610. [table redacted] Lines meeting on the Line of Zero Atomic Weight at Oscillation-Frequency 52450. [table redacted] The mean of all these results is 224*89. The atomic weigh of radium as determined by Madame Curie is 225.