624 



SCIENCE 



[N. S. Vol. XXXIV. No. i 



occurred receives confirmation from the be- 

 havior of iodine in dilute solution in glacial 

 acetic acid. If the color of this solution is 

 observed in the cold it is seen to be brown, 

 resembling in color the aqueous solution. 

 If the solution be now heated to the boil- 

 ing-point, the color changes to pink, which 

 may be taken to indicate that the com- 

 pound of iodine and acetic acid which is 

 stable at the ordinary temperature becomes 

 to a large extent dissociated at 100°. 



Now, as I have said, a general theory of 

 solution must be applicable to all classes of 

 solution, and herein lies the importance of 

 van't Hoff's osmotic pressure theory. It 

 applies equally to mixtures of gases, to mix- 

 tures of inert liquids, and to mixtures such 

 as those of sulphuric acid and water; and 

 it has the further advantage that so long 

 as the solutions considered are dilute there 

 are simple relations connecting the osmotic 

 pressure with other easily measurable prop- 

 erties of the solutions. It has been unfortu- 

 nately the custom to oppose the osmotic 

 pressure theory of solution to the hydrate, 

 or more generally the solvate, theory, in 

 which combination between solute and sol- 

 vent is assumed. The solvate theory is, in 

 the first place, not a general theory, and in 

 the second place it is perfectly compatible 

 with the osmotic pressure theory. It is in 

 fact with regard to a general theory of solu- 

 tions on the same plane as the electrolytic 

 dissociation theory of Arrhenius. This 

 theory of ionization applies to a certain 

 class of solutions, those, namely, which con- 

 duct electricity, and is a welcome and 

 necessary adjunct in accounting for the 

 numerical values of the osmotic pressure 

 found in such solutions. Similarly the hy- 

 drate, 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 concen- 

 trated solutions in which the affinity be- 

 tween 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 cal- 

 culation 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 dissocia- 

 tion theory of Arrhenius, developed inde- 

 pendently of the osmotic pressure theory 

 of van't Hoff, gives the necessary comple- 

 ment, and for aqueous solutions offers a 

 simple basis for calculation. Van't Hoff 

 has given to science the numerically defin- 

 able conception of osmotic pressure; Ar- 

 rhenius has contributed the numerically 

 definable conception of coefficient of activity 

 of electrolytes in aqueous solution, or what 

 is now called the degree of ionization. 



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 thermo- 

 dynamic method would ever have arrived 

 at the results obtained by van't Hoff 

 through osmotic pressure. Van't Hoff was 



