Atomic Weights of analogous Elements. 319 



pounds of these bodies differ progressively in properties — the 

 boiling-point for instance — and they occur together in the pro- 

 cesses of their preparation. Methyle is intermediate in chemical 

 characters between hydrogen and sethyle, and has the interme- 

 diate atomic weight ——^ — =15. If we did not know in what 



respect the one quasi-metal differed from another, we should 

 have a series of bodies precisely analogous to the metals of the 

 alkalies : but we do know it ; we know that methyle is hydrogen 

 plus a certain increment C^ H^ ; sethyle is hydrogen plus twice 

 C^H^, &c. The general expression for any such homologous 

 series is, taking x as the increment, — 



a; a-\-x; a + 2x; a + Zx, &c. 



This will equally apply to any other cases of the addition of 

 increments — the conjugate organic acids^ such as formo-benzoic 

 acid, or the series phosphoric acid PO^, 71, azophosphoric 

 acid P^NO^ 116, and deutazophosphoric acid P^N^O^ 161. 

 Now it is precisely in like manner that I regard these series 

 of elements : 1 view sodium as lithium plus a certain incre- 

 ment ; in fact, Na = L + ^, and K = L + 2^. Lithium is here the 

 starting-point, the ^^ hydrogen'' of the series; and so in like 

 manner are calcium, chlorine, and sulphur. We do not know 

 what the increments are, but we know their atomic weights. 



In the lithium series it is 16-3 > 



... calcium ... 24*2 ^ *'" 



... chlorine ... 45 "8 

 ... sulphur ... 24*1 



It is remarkable that the increments of the calcium and 

 sulphur series are the same in weight — 24, and that the incre- 

 ment of the lithium series should be almost exactly two-thirds of 

 that number. 



Why should this numerical relation always give us triads ? 

 As yet we have no instance of a fourth member of one of these 

 series, unless indeed we view the titanium series in this light ; 

 but the advance of science may furnish us with such instances if 

 my theory of increments be correct. Whether any element not 

 contained in these groups have the atomic weight of some other 

 plus a certain increment, cannot be known until we have a third 

 member of the series to prove the fact. Thus we can only 

 speculate upon the curious circumstance, that zinc and cad- 

 mium — two similar metals occurring together in nature — have 

 atomic weights differing almost by the remarkable number 24, 

 Zn 32-6 -f 23-4 = 56 at. wt. of Cd. 



