CHAPTER XIII. 



OSMOSIS : ABSORPTION OF WATER. 



I. Osmosis. — When a bladder filled with a solution of sugar has 

 the opening into it tightly tied with string and then placed in a 

 vessel full of pure water it is found that a considerable amount 

 of the latter soon passes through the walls of the bladder into 

 the interior and mixes with the sugar-solution, in spite of the 

 fact that no visible openings are present through which the 

 water travels. 



The result of this inward transference of water is that an cut- 

 ward pressure is set up within the bladder and it becomes more 

 and more distended, just as it would be if water or air were 

 forced into it mechanically. The amount of internal pressure 

 set up under these circumstances depends upon the amount 

 of sugar dissolved in the sugar-solution and also upon the 

 temperature at which the experiment is made : with a con- 

 centrated solution a greater pressure is produced than when a 

 weak solution is used, and at a high temperature the pressure is 

 greater than at a lower one. 



.Similar internal pressure tending to expand the bladder is 

 observable when solutions of potassium nitrate, copper sulphate, 

 and many other substances are used instead of sugar solution. 

 Each of these soluble compounds possesses a different power of 

 attracting water through the walls of the bladder ; the pressure 

 set up by a solution of say one per cent, of sugar is not the 

 same as that induced by a solution of one per cent, of potassium 

 nitrate. 



In these experiments it will be found that while pure water 



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