298 Hartiry—The Action of Heat on the Absorption Spectra and 
anhydrous compounds are less soluble in such a liquid as alcohol than the corre- 
sponding hydrated salts; hence more light is transmitted through the same thickness 
of solution. 
Solutions of Anhydrous Salts. 
In those cases where a salt is anhydrous its solution is but slightly altered by 
rise of temperature, if it be altered at all; and such alteration is, as a rule, in the 
nature of that which would be caused by the concentration of the solution. In 
other words, it is slightly darkened by the absorption bands being intensified, or in 
some cases widened. This effect has been already noticed, and measured in salts 
of didymium. 
Reactions of Salts at different Temperatures. 
It is worthy of remark that substances which exhibit widely different spectra 
at different temperatures also give different reactions at these temperatures. In 
salts of chromium, the changes are particularly well marked; and they indicate a 
change in the constitution of the molecules of the salts which is not a simple 
dissociation of water-molecules. The thermo-chemical studies of Recoura have 
explained this action. 
II. Concruston.—Jn any series of salts which are anhydrous, and which do not form 
well-defined crystalline hydrates, the action of heat up to the temperature of 100° C. does 
not cause any further alteration in their absorption spectra beyond that which is usual 
with substances which undergo no chemical change by such rise of temperature. The change 
is usually an increase in the intensity of the absorption or a slight widening of the absorption 
bands. 
It may here be explained that the increase in the intensity of the absorption 
does not necessarily alter the wave-lengths or the oscillation-frequencies of the 
absorbed rays; but it is to be explained by the increase in amplitude of the 
vibrations, whether these be molecular or of intramolecular origin. This is exactly 
the effect that might be expected to arise from the action of heat. Where the 
bands are slightly widened, and therefore the wave-lengths of the rays absorbed 
are necessarily slightly altered, the effect is precisely what is seen when using a 
more concentrated solution. 
Ill. Conciusion.-—As a rule crystallized metallic salts, in which water is an integral 
part of the molecule, dissolve in water at ordinary temperatures without dissociation of the 
molecule. 
No more striking instances of the molecule remaining unchanged when it 
enters into solution can be quoted than those of the hexa-hydrated cobalt chloride ; 
