DIFFUSION AND OSMOSIS 47 



with isotonic solutions there will he a transfer of the solutes 

 if these are of different chemical constitution. Take, for 

 instance, two isotonic solutions, one of salt and another of 

 sugar. When these are brought into contact there is no 

 transference of water from one solution to the other, but 

 there is a transference of the solutes. In the salt solution 

 the osmotic pressure of the sugar is zero. Hence the difference 

 of osmotic pressure of the sugar in the two solutions will 

 cause the molecules of sugar to diffuse into the salt solution. 

 For the same reason the salt will diffuse into the sugar 

 solution. 



A disregard of this fact, that a solute will always pass 

 from a solution where its osmotic pressure is high, into on 

 where its osmotic pressure is low, is a frequent source of 

 error. Thus it is said to be contrary to the laws of 

 osmosis that solutes should pass from the blood, with its 

 low osmotic pressure, into the urine, where the general osmotic 

 pressure is higher; the more so because in consequence of 

 the exchange the osmotic pressure of the urine is still 

 further increased. Such an exchange, it is argued, is contrary 

 to the ordinary laws of physics, and can therefore only be 

 accomplished by some occult vital action. This, however, is 

 not the fact, as is proved by experiment. 



Consider an inextensible osmotic cell containing a solution 

 of sugar, the walls of the cell being impermeable to sugar 

 but permeable to salt. Let us plunge such a cell into a 

 solution of salt, which has a lower osmotic pressure than 

 the sugar solution. Since the walls of the cell are inex- 

 tensible, the quantity of water in the cell cannot increase. 

 The salt, however, will pass into the cell, since the osmotic 

 pressure of the salt is greater on the outside than on the 

 inside, and the walls are permeable to the molecules of salt. 

 This passage will continue until the osmotic pressure of the 

 salt is equal inside and outside the cell; at the same time 

 the total osmotic pressure within the cell will have increased, 

 in spite of its being originally greater than the osmotic 

 pressure outside. 



Plasmolysis. — We all know that a cut flower soon dries 



