298 Hartley — The Action of Heat on the Abso7'ption Spectra and 



anhydrous compounds are less soluble in sucli 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 tlie solution. In 

 otlier 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. Conclusion. — In 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 tvhich 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 amj^litude 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. 



III. Conclusion. — 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 ; 



