A TEXTBOOK OF PHYSIOLOGY 



mate has been added. The gelatin forms a film over the end of t he- 

 tube, which is allowed to dry in the light. It is then soaked in water 

 to remove the dichromate. Copper sulphate is now placed in the tube, 

 which is then immersed in potassium ferrocyanide. By this means 

 a brown film of copper ferrocyanide becomes deposited in the almost 

 colourless gelatin, and a membrane is obtained which is good for demon- 

 stration purposes. The membrane of copper ferrocyanide has been 

 found to be impermeable to substances such as cane-sugar and dex- 

 trose, but permeable to water. It is therefore semi-permeable in 

 regard to water and such substances in solution. This can be demon- 

 strated by filling the vessel bearing the membrane with cane-sugar 

 solution, and cementing into it a rubber cork carrying a long glass tube. 

 On immersing the pot in a vessel of water, liquid is seen to rise in the 

 glass tube, and it attains a considerable height if the membrane be 

 sufficiently well made and strong. Eventually the height of the 

 column balances the pressure, which is tending to force the water in, 

 thereby giving a measure of the driving force, which, although opposite 

 to it in direction, is equivalent to the osmotic pressure of the substance 

 in solution. 



In the animal and plant world we meet with many such semi- 

 permeable membranes. Such a one is that covering peas, beans, or 

 barley grains. If the last be placed in an aqueous solution of sul- 

 phuric acid, the water penetrates the grain, which swells in consequence 

 and increases 76 per cent, of its weight. Sulphuric acid does not 

 penetrate, as is shown by the fact that the blue pigment in the aleurone 

 granules inside the grain is not changed red, as it would be if the acid 

 penetrated the grain. When the covering of the grain is broken, the 

 change of colour at once takes place. 



Instead of the acid, a salt such as sodium chloride might be used. 

 The amount of water absorbed by the seeds will then depend upon 

 the concentration of the salt in the water, since there is now competi- 

 tion between the seed and the salt for the water. Thus, the increase 

 of weight of the seeds with a 2 per cent, solution of sodium chloride 

 is about 40 per cent.; with a saturated solution it is but 14 per cent. 

 The phenomenon is, however, not one of osmosis only, the process 

 known as imbibition also comes into play. This comparative im- 

 permeability of the outer coat of seeds is recognized by agriculturists ; 

 otherwise such poisons as copper sulphate could not be used to destroy 

 fungus spores upon the seeds without killing the seeds themselves. 



Much study has been devoted to the phenomena of osmosis. It 

 has already been stated that, if the force of attraction between solvent 

 and solution be measured, the osmotic pressure of the solution is 

 measured at the same time. The exact nature of this force is not yet- 

 completely understood, but it has been shown that it is governed by 

 certain fundamental laws closely allied to those already given for gases. 

 Thus, it is found that the osmotic pressure exerted by a given quantity 

 of the dissolved substance is inversely proportional to the volume of the 

 solution (e.g., Boyle's law). In other words, the osmotic pressure of 

 a solution is proportional to the concentration of the dissolved sub- 



