422 Dr. Gladstone on the Colour of Salts in Solution. 
Chloride of Cobalt also varies in colour according to the state 
of dilution. The addition of water increases the transmissibility 
of both the yellow and the violet rays. See fig. 12 
IopIDEs. 
Iodine-water is too weak to show much absorption when placed 
in my hollow wedge, but an alcoholic solution of iodine gives the 
spectrum represented in fig. 14. 
Iodide of Gold and Potassium gives a prismatic appearance 
resembling that of iodine. The remark made in respect to the 
terbromide of gold applies in this case also. 
Iodide of Platinum and Potassium is of an intense red, and 
presents much the same prismatic appearance as the gold-salt ; 
but while the light near F is wholly absorbed, it transmits some 
blue and more violet, which certainly iodine itself does not. See 
fig. 22. 
Iodide of Palladium and Potassium is also intensely red, and 
resembles the corresponding platinum salt, except that the violet 
is not transmitted so freely as the blue. 
Sesqui-iodide of Iron is of a deep red colour, and gives a spec- 
trum resembling that of iodine. 
Iodide of Nickel, when dissolved in a little water, is of a dark 
green, and affords a spectrum closely resembling that of iodine, 
except that the red ray is gradually absorbed by the metal. See 
fig. 18. When water is added to this, the same amount of salt 
transmits more and more blue, and assumes the appearance of 
fig. 10, a compound of nickel with a colourless acid. 
Iodide of Cobalt, in saturated solution, is of a dark green, pass- 
ing into a very deep red as the stratum increases. On dilution 
it becomes pinker. 
Double Salts. 
When the two bases in a double salt are both coloured, a 
similar result is obtained. 
Double Chloride of Copper and Platinum is a definite salt, form- 
ing green crystals. A saturated solution of these gives the spec- 
trum represented in fig. 24; but on addition of water, the same 
amount of salt transmits a somewhat broader band of green, and 
more blue, giving then the appearance of fig. 20, with the red 
ray gradually absorbed. This remarkable compound therefore 
shows, when in strong aqueous solution, the absorption due to 
each of its three constituents,—copper, platinum, and chlorine, 
see figs. 20 and 21; though, on dilution, the absorption due to 
chlorine is somewhat modified, as in the case of the uncombined 
copper-salt. 
These results show that each coloured constituent of a salt 
