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312 Hartiey—The Action of Heat on the Absorption Spectra, &c. 
individual particle, is very inadequately represented by our usual chemical formule, 
since these serve merely to symbolise certain well-known chemical reactions, but 
fail to express any relation between physical and chemical properties, or the 
dynamic conditions of the molecule. But as certain molecular groupings are 
characterised by the absorption of rays of particular wave-lengths (absorption 
bands), it is evidently possible to draw conclusions as to the constitution of 
substances from their absorption spectra. 
CoNCLUSIONS. 
I. When a definite crystalline hydrate dissolves in a solvent which is not water, and 
is without chemical action upon it, the molecule of the salt remains unchanged in chemical 
composition. 
II. 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 
Jurther alteration i 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 
inerease im the intensity of the absorption, or a slight widening of the absorption bands. 
Ill. As a rule, crystalline metallic salis in which water is an integral part of the 
molecule, dissolve in water at ordinary temperatures, without dissociation of the molecule. 
IV. Crystallized hydrated salts dissolved in a minimum of water at 20° C. undergo 
dissociation by rise of temperature. The extent of the dissociation may proceed as far as 
complete dehydration of the compound, so that more or less of the anhydrous salt may be 
formed in the solution. 
V. The most stable compound which can exist in a saturated solution at 16° or 20° ts 
not always of the same composition as the molecule of the crystallized solid at the same 
temperature, since the solid may undergo partial dissociation from its water of crystal- 
lization when the molecule enters into solution. 
VI. Saturated solutions of deliquescent salts combine with water when diluted to 
constitute molecules of more complex hydrated compounds in such solution. 
VII. When a saturated solution of a coloured salt undergoes a great change of colour 
upon dilution, or any remarkable change in tts absorption spectrum due to the same 
cause, the dilution is always accompanied by a considerable evolution of heat. 
