THE BEARING OF OSMOTIC PRESSURE 



83 



tilp laboratory — not to those solutions which give us chemistry, 

 biology, etc. Such solutions were in general found to exert os- 

 motic pressures that are too great even if we take into account 

 the dissociation of such solutions. And here again there was for 

 a time considerable trouble. We had a theory of solution which 

 might be called a theory of ideal conditions which were of very 

 little use in science — a theory which did not apply to the very 

 cases in which men of science were most interested. 



We think that we have now a fairly satisfactory explanation 

 of the apparently abnormal results presented by solutions of 

 moderate and of great concentration. The solvate theory of 

 solution says that a dissolved substance combines with more or 

 less of the solvent, and this theory must be regarded merely as 

 supplementing the theory of electrolytic dissociation. The dis- 

 solved substances combining with more or less of the solvent 

 remove it from the field as part of the solvent. These solutions 

 are concentrated by just so much solvent as is combined with the 

 dissolved substances, and are in reality more concentrated than 

 we would suppose them to be from the amount of substance dis- 

 solved in them. This conclusion seems to be justified by the work 

 of this and other laboratories during the past fifteen years. 

 Indeed, more than a dozen independent lines of experimental 

 evidence all pointing to the correctness of the solvate theory of 

 solutions have been brought to light in this laboratory alone dur- 

 ing the past ten years. 



Without going into any detail here on this point, suffice it to 

 say that when the theory of electrolytic dissociation is supple- 

 mented by the solvate theory of solution, which enables us to 

 calculate the amount of solvent combined with the dissolved 

 substance — to calculate the true concentration of the solution — 

 we can then calculate the osmotic pressure of fairly concentrated 

 solutions approximately, knowing the amount of their dissociation. 

 In a word, we have today a theory of the nature of the real solu- 

 tions that we use in biology and in chemistry, and we have already 

 seen the meaning of such a theory for the biological sciences in 

 general. 



