DISCUSSION OF EVIDENCE. 189 



The transmission curves obtained seem to justify the conclusion that 

 combined water has less power to absorb light than uncombined. We 

 have been able to find no other rational explanation which would 

 account satisfactorily for our results. The difference in the behavior 

 of hydrated and non-hydrated salts seems unquestionable. 



Any attempt to explain such a difference as the above on the ground 

 of a change in the dielectric constant of the medium does not appear to 

 have a good physical basis. Why the presence of the one class of 

 salts alters the dielectric constant of the medium differently from the 

 other class, is a question that would have to be answered. This 

 attempt to explain our results does not appear to be much more than 

 words. We regard, then, the spectroscopic evidence in its bearing on 

 the solvate theory of solution as among the most important. The 

 presence of definite " sol vent bands" in the different solvents and the 

 difference between the absorption of aqueous solutions of non-hydrated 

 and strongly hydrated salts are to be counted as among the strongest 

 and most direct lines of evidence thus far brought to light in my labora- 

 tory bearing on the solvate theory of solution. 



SUMMARY OF THE LINES OF EVIDENCE OBTAINED IN THIS LABORATORY BEARING 



ON THE SOLVATE THEORY OF SOLUTION. 



The following lines of evidence bearing on the solvate theory of 

 solution have, then, been established in this laboratory. 1 



1. Relation between lowering of the freezing-point of water and 

 water of crystallization of the dissolved substance. 



2. Approximate composition of the hydrates formed by various 

 substances in solution. 



3. Relation between the minima in the freezing-point curves and 

 the minima in the boiling-point curves. 



4. Relation between water of crystallization and temperature of 

 crystallization. 



5. Hydrate theory in aqueous solutions becomes the solvate theory 

 in solutions in general. 



6. Temperature coefficients of conductivity and hydration. 



7. Relation between hydration of the ions and their ionic volumes. 



8. Hydration of the ions and the velocities with which they move. 



9. Dissociation as measured by the freezing-point method and by 

 the conductivity method. 



10. Effect of one salt with hydrating power on the hydrates formed 

 by a second salt in the same solution. 



11. Investigations in mixed solvents. 



12. Spectroscopic evidence bearing on the solvate theory of solution; 

 work of Jones and Uhler. 



13. Work of Jones and Anderson on absorption spectra, in which 

 the presence of "solvate" bands was first detected. This showed that 



J See Journ. Franklin Inst., Dec. 1913. 



