1912-13.] The Absorption of Light by Inorganic Salts, No. X. 155 
value for iodine, and A for cobalt iodide varying in the same way with 
the wave-length, but having only a value about one-fifth as large. Also 
the absorption of a cobalt iodide solution two months old was very much 
less than this. 
Our intention here was to study the effect on A of varying c for a 
wave-length sufficiently far in the violet to get the beginning of the 
characteristic absorption, but still one suitable for use with the spectro- 
photometer. The wave-length \ = 434 fxjx was selected. It was found, 
however, that different solutions gave contradictory results, the solution 
prepared from iodide that had been standing a long time in the bottle 
giving a much greater absorption. Investigation showed that this was 
due to the formation of the oxyiodide and the consequent liberation of 
free iodine. When the solution was made up, the oxyiodide being insoluble 
was removed by filtration and the iodine went into solution. 
Nickel oxyiodide has been described by Erdmann,* and the formula 
NiI 2 9Ni015H 2 0 has been fitted to the results of his analysis in Morley 
and Muir’s Dictionary of Chemistry. Cobalt oxyiodide seems, according 
to Hartley,]* to have the formula Co 2 I 3 0. 
Our results in the second, third, and fourth articles thus require correc- 
tion. Apparently the absorption of the pure iodides should not differ 
from that of the sulphates. Iodine itself absorbs so strongly, and it seems 
almost impossible to avoid a trace of iodine in the solution, so no attempt 
was made to determine again the absorption of the pure iodides. Indeed, 
the formation of the oxyiodide shows itself by the colour of the solution 
long before the precipitate has become appreciable. 
It should be noticed that in the third article we used NiI 2 6H 2 0, the 
commonly accepted formula for the usual hydrate of nickel iodide. 
Hartley J has shown it to be NiI 2 7H 2 0. 
* Journ. f. prak. Chem ., 7 , p. 254 (1836). + Journ. Chem. Soc ., 12 (1874). 
f Dublin Trans. (2), 7 , p. 264 (1900). 
(Issued separately April 22, 1913.) 
