on the Colour of Salts in Solution. 425 
them in the same direction. In the last seven instances it is 
more than probable that a temporary chemical change is effected 
in the liquid. In the preceding paper, I have shown that the 
chloride and bromide of copper, the chloride and iodide of nickel, 
the sesquichloride of iron, and one or two other analogous salts, 
give a saturated aqueous solution of a different colour to what is . 
presented by the same solution when more water is added; and 
that this change is due to the fact, that in the saturated solution 
both the halogen and the metal exert their own absorbent power 
on the rays during their passage through the liquid, while im the 
dilute solution the metal alone influences the transmission of light. 
That this phenomenon results from some difference of arrange- 
ment among the elements of the salt and water, scarcely admits 
of a doubt; yet what that difference of arrangement may be is 
not so easily determined. This chemical change it is that is 
affected by temperature, an inerease of heat having the opposite 
effect to an addition of water, anda diminution of heat having the 
same effect as dilution. Thus the light transmitted by a dilute 
solution of chloride of copper at the ordinary temperature, when 
analysed by a prism, shows the spectrum represented in Plate IT, 
fig. 9, but on heating the solution, an absorption due to chlorine 
manifests itself, and the coloured band is reduced to the dimen- 
sions of fig. 21: and similarly, the bromide of copper, if dilute and 
cold, presents the ordinary prismatic appearance of copper salts, 
fig. 9; but if the same liquid be heated, it assumes the green 
colour and the modified spectrum of the saturated solution, 
fig. 17, in which the bromine exerts its absorbent power. In 
each case the influence of the halogen disappears as soon as the 
heat is withdrawn ; and that this is not confined to the range of 
temperature above what we designate as ordinary, was proved by 
exposing a green solution of chloride of copper to a freezing mix- 
ture, when it assumed a distinctly bluish tint. 
The case of the cobalt salts is evidently analogous: the dilute 
sulphocyanide of cobalt in water, or the chloride in aqueous al- 
cohol, gives a prismatic appearance similar to, but not identical 
with, that represented in fig. 12; but when heated, additional 
dark spaces show themselves; the more refrangible red and less 
refrangible orange rays are wholly absorbed, and the more re- 
frangible portion of the orange is allowed to penetrate but to a 
short distance, while a perfectly black line shows itself coincident 
with D, but somewhat broader. In fact the hot cobalt salt 
gives precisely the same prismatic appearance as that given by 
smalt-blue glass, or by an alcoholic solution of cobalt, as figured 
in the plate annexed to my paper “ On the Use of the Prism in 
Qualitative Analysis,” in the Quarterly Journal of the Chemical 
Society, April 1857, 
