Colour and Chemical Constitution. 
49 
In the case of cobalt the homologous lines x\ 621, 662 and 694, seen 
respectively in the acid chloride, bromide, and iodide, have ratios which are 
almost exactly the same as the 8th root of the ratios of the molecular 
weights. 
In the phthaleins, where the substituents are singly-linked carbon atoms, 
it is the 4th root of the ratio of the molecular weights which agrees w^ith the 
ratio of the absorption wave-lengths, but the halogen- derivatives do not 
follow this rule. 
I do not yet know if it is more than a coincidence that these roots, 6, 8 
and 4, are the same as the valencies (periodic place) of uranium cobalt and 
carbon. 
Note on Uranous (Tetravalent) Compounds. 
These are blue-green and have a striking spectrum. The band-centres 
of the + + + ion are : XX 688 and 673 (hair-lines), 650 (very strong), 553, 
496 and 440. U(P03)^ in solid HPO^ has the bands XX ? 700, 689, 670 
(strong), 629, ? 580 and 545. 
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