94 PHYSICAL SCIENCE 



of physical conceptions to the problems of living 

 matter chiefly depends on the knowledge we 

 possess of the physics and chemistry of ordinary 

 solutions. 



The earliest Investigations of the subject were 

 of a chemical nature, and, till the passage of 

 electric currents through liquids came to be 

 examined at the beginning of the nineteenth 

 century, little systematic study of the physical 

 properties of solutions was made. But since that 

 period there has been constant progress, and many 

 new fields of research have been opened up. 



It happens constantly that light Is thrown 

 on the dark places of one science by work 

 undertaken to elucidate those of another ; and, 

 in this case, the starting-point for the modern 

 theory of solution Is found In some experiments 

 made by Pfeffer in 1877 In a botanical laboratory. 

 Ten years earlier, Traube, In studying the modes 

 of formation of the organic cells of plants and 

 animals, had discovered how to construct artificial 

 membranes permeable to water but not to solu- 

 tions of certain substances dissolved therein. 

 Pfeffer made a further examination of these 

 semi-permeable membranes, as they have been 

 called, and by their use obtained results of great 

 importance in the study of biology. 



A porous pot of unglazed earthenware, 6 to 

 8 centimetres high and 2 or 3 centimetres In 

 diameter, Is sealed by means of sealing-wax to 

 a glass tube, as shown in Fig. 25. Having been 

 thoroughly washed. It Is filled with the solution 

 of a salt, such as potassium ferrocyanlde, and 

 the outside is then surrounded with the solution 



