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to a general theory of solutions on the same plane as the electrolytic dissocia- 

 tion theory of Arrhenius. This theory of ionisation applies to a certain class 

 of solutions, those, namely, which conduct electricity, and is a welcome and 

 necessary adjunct in accounting for the numerical values of the osmotic pressure 

 found in such solutions. Similarly the hydrate, or more generally the solvate, 

 theory is applicable only to those solutions in which combination between solvent 

 and solute occurs, and will no doubt in time afford valuable information with 

 regard to the osmotic pressure, especially of concentrated solutions in which the 

 affinity between solvent and solute is most evident. It can tell us nothing 

 about solutions in which one, or both, components is inactive, just as the 

 electrolytic dissociation theory can tell us nothing about solutions which do not 

 conduct electricity. 



The great practical advantage bequeathed to chemists by the genius of van't 

 Hoff is the assimilation of substances in dilute solution to substances in the 

 gaseous state. Here all substances obey the same physical laws, and a secure 

 basis is offered for calculation connecting measurable physical magnitudes, 

 irrespective of the chemical nature of the substances and of the solvents in which 

 they are dissolved, provided only that the solutions are non-electrolytes. If 

 the solutions are electrolytes, the dissociation theory of Arrhenius, developed 

 independently of the osmotic pressure theory of van't Hoff, gives the necessary 

 complement, and for aqueous solutions offers a simple basis for calculation. 

 Van't Hoff has given to science the numerically definable conception of osmotic 

 pressure ; Arrhenius has contributed the numerically definable conception of 

 coefficient of activity of electrolytes in aqueous solution, or what is now called 

 the degree of ionisation. 



Of late there has been a tendency in some thermodynamical quarters to 

 belittle the importance of the conception of osmotic pressure. It is quite true 

 that from the mathematical thermodynamical point of view it may be relegated 

 to a second place, and even dispensed with altogether, for it is thermodynamically 

 related to other magnitudes which can be substituted for it. But it may be 

 questioned if without the conception the cultivators of the thermodynamic 

 method would ever have arrived at the results obtained by van't Hoff 

 through osmotic pressure. Van't Hoff was only an amateur of thermodynamics, 

 but the results achieved by him in that field are of lasting importance, and his 

 work and the conception of osmotic pressure have given a great stimulus to the 

 cultivation of thermodynamics to chemistry. 



And here we trench on a question on which a certain confusion of thought 

 often exists. To the investigator it is open to choose that one of several equiva- 

 lent methods or conceptions which best suits his personal idiosyncrasy. To the 

 teacher such a choice is not open. He must choose the method or conception 

 which is most clearly intelligible to students, and is at the same time least likely 

 to lead to misconception. Osmotic pressure is a conception which the chemical 

 student of mediocre mathematical attainments can grasp, and it is not difficult to 

 teach the general elementary theory of dilute solutions by means of it and of 

 reversible cycles without liability to radical error or misconception. I should be 

 sorry on the other hand to try to teach the theory of solutions to ordinary 

 chemical students by means of any thermodynamic function. The two methods 

 are thermodynamically equivalent, and the second is mathematically more ele- 

 gant and in a way simpler, but it affords less opportunity than the first for the 

 student to submit his methods to any practical check or test, and in nine r-ases 

 out of ten would lead to error and confusion. The difficulty of the student is 

 not the mathematical one; with the excellent teaching of mathematics now 

 afforded to students of physics and chemistry the mathematical difficulty has 

 practically disappeared — the difficulty lies in critically scrutinising the conditions 

 under which each equation is used is applicable. 



Of the mechanism of osmotic pressure we still know nothing, but with the 

 practical measurement of osmotic pressure great advances have been made in 

 recent years. In particular the admirable work of Morse and Frazer is of the 

 first importance in establishing for solutions up to normal concentration the 

 relationship between osmotic pressure and composition, and its variation with 

 the temperature. Much may be anticipated from the continuation of these 

 accurate and valuable researches, the experimental difficulties of which are 

 enormous. 



