Diff. Diff. Diff. Diff.
Mg = 12 P = 81 O = 8
⎬8 ⎬44 ⎬8
Li = 7 Ca = 20 As= 75 S = 16
⎬16 ⎬3 × 8 ⎬44 ⎬3 × 8
Na = 23 Sr = 44 Sb = 119 Se = 40
⎬16 ⎬3 × 8 ⎬2 × 44 ⎬3 × 8
K = 39 Ba = 68 Bi = 207 Te = 64

and pointed out some really striking relationships, such as the following:—

F = 19.
Cl = 35·5 = 19 + 16·5.
Br = 80 = 19 + 2 × 16·5 + 28.
I = 127 = 2 x 19 + 2 × 16·5 + 2 × 28.

A. Strecker, in his work ‘Theorien und Experimente zur Bestimmung der Atomgewichte der Elemente’ (Braunschweig, 1859), after summarising the data relating to the subject, and pointing out the remarkable series of equivalents—

Cr = 26·2 Mn = 27·6 Fe = 28 Ni = 29 Co = 30 Cu = 31·7 Zn = 32·5

remarks that: ‘It is hardly probable that all the above-mentioned relations between the atomic weights (or equivalents) of chemically analogous elements are merely accidental. We must, however, leave to the future the discovery of the law of the relations which appears in these figures.’[1]

In such attempts at arrangement and in such views are to be recognised the real forerunners of the periodic law; the ground was prepared for it between 1860 and 1870, and that it was not expressed in a determinate form before the end of the decade may, I suppose, be ascribed to the fact that only analogous elements had been compared. The idea of seeking for a relation between the atomic weights of all the elements was foreign to the ideas then current, so that neither the vis tellurique of De Chancourtois, nor the law of octaves of Newlands, could secure anybody's attention. And yet both De Chancourtois and Newlands like Dumas and Strecker, more than Lenssen and Pettenkofer, had made an approach to the periodic law and had discovered its germs. The solution of the problem advanced but slowly, because the facts, but not the law, stood foremost in all attempts; and the law could not awaken a general interest so long as elements, having no apparent connection with each other, were included in the same octave, as for example:—

1st octave of NewlandsHFClCo & NiBrPdIrPt & Ir
7th DittoOSFeSeRh & RuTeAuOs or Th

Analogies of the above order seemed quite accidental, and the more so as the octave contained occasionally ten elements instead of eight, and when two such elements as Ba and V, Co and Ni, or Rh and Ru, occupied one place in the octave.[2] Nevertheless, the fruit was ripening, and I now see clearly that Strecker, De Chancourtois, and Newlands stood foremost in the way towards the discovery of the periodic law, and that they merely wanted the boldness necessary to place the whole question at such a height that its reflection on the facts could be clearly seen.

A third circumstance which revealed the periodicity of chemical elements was the accumulation, by the end of the sixties, of new information respecting the rare elements, disclosing their many-sided relations to the other elements and to each other. The researches of Marignac on niobium, and those of Roscoe on vanadium, were of special moment. The striking analogies between vanadium and phosphorus on the one hand, and between vanadium and chromium on the other, which became so apparent in the investigations connected with that element, naturally induced the comparison of V = 51 with Cr = 52, Nb = 94 with Mo = 96, and Ta = 192 with W = 194; while, on the other hand, P = 31 could be compared with S = 32, As = 75 with Se = 79, and Sb = 120 with Te = 125. From such approximations there remained but one step to the discovery of the law of periodicity.