The Organisation of Matter 



and then cautiously add some coloured alcohol, 

 or, more simply, some wine, this liquid, being 

 lighter, forms at first a distinctly separate layer 

 on the surface of the water, just as oil would do, 

 with the difference that whereas the oil would 

 remain permanently on top of the water, the 

 alcohol gradually mixes with it, and being 

 coloured enables this gradual diffusion to be 

 followed. The water, on its side, travels in 

 the opposite direction and diffuses into the 

 alcohol. All miscible liquids exhibit similar 

 phenomena. If the liquids are separated by a 

 permeable membrane the diffusion is known 

 by the name of osmosis. Thus in 1748 Abbe" 

 Nollet studied the osmotic exchanges between 

 water and alcohol separated by a pig's bladder. 

 The experiment may be made with substances 

 dissolved in liquids, and it is then found that 

 the bodies in solution traverse the membrane 

 at very different speeds, according to their 

 nature. If, for example, on one side of this 

 membrane there is pure water and on the other 

 an aqueous solution of sugar and molasses, 

 the sugar, a crystallisable body, will be seen 



to cross the partition rapidly, whereas the non- 

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