RELATION OF WATER TO THE PROTOPLASM 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. Withany 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, ag 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 
