APPENDIX 



239 



H 



A 



B 



FIG. 



the molecules of certain substances dissolved in the water. But very few or 

 no membranes are equally permeable to water and to molecules of the 

 substances dissolved in the water. If in the accompany- 

 ing diagram the compartment A is filled with pure water, 

 and B with a sodium chloride solution, the liquids in 

 the two compartments will ultimately be found to be 

 equal in bulk as they were at the start, and each will 

 be a solution of salt of half the original strength of 

 that in the compartment B But at first the volume 

 of the liquid in compartment B increases, because more 

 water molecules pass into it from A than salt mole- 

 cules pass from B into A. The term osmosis is generally 

 limited to the stream of water molecules passing 

 through a membrane, while the term dialysis is applied 

 to the passage of the molecules in solution in the 

 water. The osmotic stream of water is especially 

 important, and in connection with this it is necessary 

 to explain the term osmotic pressure. At first, then, 

 osmosis (the diffusion of water) is more rapid than the dialysis (the diffusion 

 of the salt molecules or ions). The older explanation of this was that salt 

 attracted the water, but we now express the fact differently by saying that 

 the salt in solution exerts a certain osmotic pressure : the result of the 

 osmotic pressure is that- more water flows from the water side to the side of 

 the solution than in the contrary direction. The osmotic pressure varies 

 with the amount of substance in solution, and is also altered by variations of 

 temperature, occurring more rapidly at high than at low temperatures. 



If we imagine two masses of water separated by a permeable membrane, 

 as many water molecules will pass through from one side as from the other, 

 and so the volumes of the two masses of water will remain unchanged. If 

 now we imagine the membrane M is not permeable except to water, and the 

 compartment A contains water, and the compartment B contains a solution 

 of salt or sugar; in these circumstances water will pass through into B, 

 and the volume of B will increase in proportion to the osmotic pressure of 

 the sugar or salt in solution in jB, but no molecules of sugar or salt can get 

 through into A from B, so the volume of fluid in A will continue to decrease, 

 until at last a limit is reached. The determination of this limit, as measured 

 by the height of a column of fluid or mercury which it will support, will give 

 us a measurement of the osmotic pressure. Membranes of this nature are 

 called semi-permeable. One of the best kinds of semi-permeable membrane is 

 ferrocyanide of copper. This may be made by taking a cell of porous 

 earthenware and washing it out first with copper sulphate and then with 

 potassium ferrocyanide. An insoluble precipitate of copper ferrocyanide is 

 thus deposited in the pores of the earthenware. If such a cell is filled with 

 a 1-per-cent. solution of sodium chloride, water diffuses in till the pressure 

 registered by a manometer connected to it registers the enormous height of 

 5,000 millimetres of mercury. Theoretically it is possible to measure osmotic 

 pressure by a manometer in this way, but practically it is seldom done, and 

 some of the indirect methods of measurement described later are used 



