312 Hartley — The Action of Heat on the Absorption Spectra, 6cc. 



individual particle, is very inadequately represented by our usual chemical formulae, 

 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 ivhich is not tvater, and 

 is without chemical action upon it, the molecule of the salt remains unchanged in chemical 

 composition. 



II. In any series of salts tvhich are anhydrous, and ivhich 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 ahsorption spectra, heyond that which is usual tvith 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. 



III. As a rtile, crystalline metallic salts in tvhich ivatcr is an integral part of the 

 molecule, dissolve in water at ordinary temperatures, ivithout 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° is 

 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 ivJien the molecule enters into solution. 



VI. Saturated solutions of deliquescent salts combine tvith water tuhen diluted to 

 constitute molecides of more complex hydrated compounds in such solution. 



VII. Wlien a saturated solution of a coloured salt undergoes a great change of colour 

 upon dilution, or any remarkable change in its absorption spectrum due to the sa)ne 

 cause, tlie dilution is ahvays accompanied by a considerable evolution of heat. 



