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series, and there is no other place for an element with the 
chemical and physical properties of chromium vacant in the 
table. For like reasons the positions in the series of 
molydenum and tungsten (the analogues of chromium) are 
also determined. By assigning to chromium the constitu- 
tion 9 H6, it forms a triad with the missing elements 
a; = 36, and x- IS, which are within a unit, the atomic 
weights of fluorine and chlorine. 
In the arrangement of the elements which I have classified 
as Wjn, little assistance is derived from known analogies, 
when nitrogen and silicium are admitted in the same series 
with the iron and platinum groups of metals ; yet, it might 
he expected that elements so abundant, and so widely dif- 
fused in nature as nitrogen, silicium, and iron, would occupy 
important positions in any rational classification of elemen- 
tary species. We have seen that the first three places in 
the preceding series H n, H2 n, H3 n, H 5n, are all occupied 
by elements with atomic weights which exclude nitrogen, 
silicium, and iron, while the latter element is excluded from 
the series H kn, and H 6n, by chromium and titanium. The 
atomic weight of N, Si, and Fe, besides being whole numbers, 
are exact multiples of 7. N, and Si, are, consequently, 
excluded from the vacant homologous positions in the series 
H4 n, H Qn. 
Since the investigation of the properties of silicium by 
Berzelius, who regarded silicic acid as a trioxide, much dis- 
cussion has arisen as to whether the atomic weight of 
silicium be 21 or 28 ; or the formula for its oxide Si0 3 , or 
Si0 2 . Chemists are now generally agreed upon the latter 
formula for silicic acid, and have accordingly classified sili- 
cium with titanium, as the oxide Si0 2 , agrees with titanic 
acid Ti0 2 . Now, if silicium were the true analogue of 
titanium, the oxides of these elements should be isomorphous, 
whereas the crystalline form of quartz is hexagonal, while 
rutile, anatase, brookite, zirconia and tinstone (similar 
