PHTSIOLOGT 



thinking deeply on this question, he met de 

 Vries, who told him the results of Pfeffer's 

 observations. Seldom has an interchange of 

 scientific ideas been more fruitful than this his- 

 toric interview. It is a classic example of cross 

 fertilization among the sciences. Not only did it 

 initiate a new era in physical chemistry, but it 

 was destined to be of fundamental importance 

 to biology. 



Pfeffer's results led van't Hoff to the revo- 

 lutionary notion that molecules of dissolved sub- 

 stances obey the laws of gases. At that time it 

 seemed possible that the application of these 

 laws might clear up at one stroke most of the 

 difficulties which had made the study of solu- 

 tions so puzzling. This idea had far-reaching 

 results. 



An important difficulty remained to be ac- 

 counted for. Inorganic salts failed to obey the 

 general law which governs the osmotic behavior 

 of sugar and many organic substances, de Vries 

 had observed this in experiments on plant cells 

 and had expressed the deviations from the law 

 by means of the so-called "isotonic coefficients." 



