Section III, 1891. [ 27 ] Trans. Roy. Soc. Canada. 



V. — On the Variation ivifJi Temperature and Concentration of the Absorption Spectra 



of Aqueous Solutions of Salts. 



By Prof. J. Gr. MacG-regor, D.Sc, Dalhousie College, Halifax, N. S. 



(Read May 29, 1891.) 



The study of auy property of a solution which is possessed only to a slight extent, or 

 not at all, by the solvent, is likely to throw light upon the constitution of the solution ; and 

 for this reason (the electrical conductivity of water being practically zero and that of solu- 

 tions of salts and acids finite and measurable) the study of electrolysis has thrown much 

 light upon the relation of water to the substance dissolved in it, and upon the many 

 physical and chemical phenomena which depend upon this relation. The selective absorp- 

 tion of light is another property which many solutions possess in a very high degree, but 

 which water possesses hardly at all. For though water has indeed a distinct absorption 

 spectrum, relatively to many solutions of coloured salts its selective absorption may be said 

 to be exceedingly small. It might be expected, therefore, that the study of the absorption 

 spectra of aqueous solutions would also throw light on their constitution, though, for 

 obvious reasons, not to such an extent as in the case of electrolysis. Nevertheless this 

 field has been but slightly cultivated, and very little is known as to the dependence of 

 the absorption spectra of solutions upon their concentration, temperature or other physical 

 property. 



My attention has been drawn to this field as a fruitful one for investigation, in fol- 

 lowing out the working hypotheses which I usually employ in thinking of problems in 

 solution. Though the hypotheses are crude, and would require to be made much more 

 precise before they could become the fundamental assumptions of a theory of solution, 

 they may be stated here. 



The molecular theory of the constitution of material bodies being assumed, the fact 

 that a drop of water can hold together in opposition to the attraction of the earth, seems 

 to imply the action of attractive forces between its molecules. The same may be said 

 of the molecules of a crystal of salt. Also the fact that a crystal of salt, which at ordinary 

 temperatures will remain for any length of time surrounded by air without changing in 

 mass, and which therefore undergoes sublimation to no appreciable extent, will, when 

 surrounded by water, diminish rapidly in mass, seems to imply the action of attractive 

 forces between the water molecules and the salt molecules. If these attractive forces be 

 assumed, a relation between their magnitudes is suggested by two other facts. The first, 

 that in the solution of a salt crystal the water molecules are able to carry off the salt 

 molecules in opposition to the attracting forces of the other salt molecules of the crystal, 

 su"-gests the assumption that, other things being equal, the attraction between salt and 

 water molecules is greater than that between salt molecules themselves. The second fact, 

 that almost all salt particles at some temperature carry water with them out of a solu- 



