202 THE BODY AT WORK 



say, of the passage of water and of solutions through mem- 

 branes are of such high importance in relation to the life of 

 the tissues that it may be permissible to make a further digres- 

 sion for the purpose of describing them (cf. pp. 40, 128). A very 

 simple apparatus will suffice to exhibit a phenomenon which 

 will give an idea of the meaning of osmosis. If the top of a 

 glass funnel, covered with a piece of bladder, so fastened to 

 its edge as to make it water-tight, be fixed in an inverted 

 position in a glass vessel, the glass vessel filled with water, and 

 the funnel filled to the same level with a solution of sugar, it will 

 soon be evident that water is passing through the membrane 

 into the funnel. The level of the sugar-solution will rise in the 

 tube of the funnel. If, instead of water outside the funnel and 

 sugar-solution inside it, a strong solution of sugar be placed in 

 the funnel and a weaker solution outside it, water will leave the 

 weaker for the stronger solution, and sugar the stronger solu- 

 tion for the weaker. If some of the solution in the funnel 

 be removed from time to time so that the pressure in it is 

 kept down to the same level as that outside it, water will 

 continue to enter through the membrane and sugar to leave 

 the contents of the funnel until the concentration of sugar is 

 the same on the two sides. The fluids will then be of identical 

 composition, and therefore isosmotic. In the further con- 

 sideration of the phenomena of osmosis, a distinction must be 

 made between permeable and hemipermeable membranes. 

 Suppose in the first instance that a permeable membrane is 

 used. Let it be so placed as to separate two watery solutions 

 of different constitution, yet of the same osmotic pressure. 

 By their being of the same osmotic pressure is meant that 

 they are of the same molecular concentration. The liquid A 

 contains certain salts in solution ; but the liquid B may con- 

 tain the same salts in quite different proportions. It so happens, 

 however, that the salts are so balanced that the total tension 

 of the salts in A is equal to the total tension of the salts in B. 

 At first there may be some change in level in the two liquids, 

 owing to differences in rates of diffusion through the mem- 

 brane of the various salts which they contain ; but after a time 

 the levels of the two liquids will be the same. To outward 

 appearance, nothing will have happened. Nevertheless, if the 

 experiment has been continued for a sufficient length of time, 



