EELATION OF WATEK TO THE PBOTOPLASM 61 



Other substances than sugar have a similar power of 

 setting up osmotic currents, which, indeed, is especially pro- 

 minent in those which are crystalloid in character, though it 

 is not confined to them. Solutions containing different sub- 

 stances in equal percentages do not, however, possess equal 

 osmotic powers ; each one has its own special ability and 

 is said to exert its particular osmotic pressure. With any par- 

 ticular substance, however, the osmotic pressure is approxi- 

 mately proportional to the concentration of the solution. 



The simple process of osmosis is capable of substantial 

 modification according to the character or composition of 

 the membrane. While a diaphragm of bladder or of veget- 

 able parchment will allow the two streams to pass through 

 it in the manner described, it is by no means unusual to 

 find membranes which offer opposition to the passage of 

 the substances which are dissolved ; indeed, the latter is in 

 some cases unable to pass through at all, the membrane 

 being impermeable to it. Such a membrane is called a 

 semipermeable membrane as far as that particular salt is 

 concerned, as it will allow the water to pass but not the 

 solution on the other side. Such a membrane is the pre- 

 cipitation membrane produced by the contact of aqueous 

 solutions of potassic ferrocyanide and copper sulphate. 

 It is difficult to prepare such a membrane for experiment, 

 but it becomes possible if a porous pot containing one of 

 the solutions is immersed nearly to the top in a vessel 

 containing the other. As the latter slowly penetrates the 

 sides of the porous pot it eventually comes into contact 

 with the solution in its interior, and a precipitation membrane 

 is formed in the substance of the walls of the pot. If such 

 a prepared pot be emptied of this solution and a solution 

 of cane sugar be poured into it, and the pot and its contents 

 be placed in water, the sugar does not pass out, though water 

 continuously passes in. The membrane is shown to be a 

 semipermeable one, permeable to water, but not to sugar 

 solution. 



But the membrane which is concerned in osmotic 



