OSMOSIS. 



101 



alcohol cannot do so at all readily, so that if a mixture of alcohol 

 and water is placed in a bladder and closely tied up, the water 

 will gradually pass through the membrane and evaporate on 

 the outside, whilst almost the whole of the spirit remains in a 

 comparatively concentrated form. The same difference is exhibited 

 by different aqueous solutions as compared with one another or 

 with pure water. 



Expt. 92. Illustration of Osmosis. Thus, if a long glass tube 

 be attached to the narrow end of a "(fell-shaped vessel, the mouth 

 of which is tied over with a piece of bladder, and the whole 

 mounted by means of a stand (fig. 52), so 

 that the bell is immersed in the fluid con- 

 tained in a larger outside vessel, it will be 

 found that if the bell be filled with brine and 

 the outer vessel with plain water, the salt con- 

 tained in the brine will gradually pass through 

 the membrane and produce a weak salt solu- 

 tion in the outer vessel; whilst, conversely, 

 plain water will pass inwards from the outer 

 vessel into the bladder, diluting the brine 

 therein, and causing the level of the fluid to 

 ascend very considerably in the glass tube, 

 indicating that much more liquid passes into 

 the bell through the bladder than in the 

 opposite direction. Phenomena connected 

 with the different degrees of facility of passage 

 of dissolved solids, &c., through membranes, 

 are referred to under the name of dialysis ; 

 whilst the action causing fluid to pass through 

 in one direction, rather than in the opposite, 

 is termed osmosis. Animal membranes are 

 not indispensable to the action, as chemically 

 prepared paper (vegetable parchment) and simi- 

 lar materials can often be used to form a septum 

 or diaphragm, permitting dialysis and osmosis 

 to take place through its body 



Substances which are readily crystallised 

 from aqueous solution, or which can be obtained crystalline by other 

 treatment (e.g., in the case of gases, intense chilling, so as first to con- 

 dense to the liquid state, and then to freeze to solids, like ice), will 

 readily pass through such membranes, so as to become diffused 

 through plain water on the other side of the membrane ; whereas 

 bodies of a glue-like or "colloid" character (Expt. 70), will not pass 

 through at all, or at most only very slowly and with much diffi- 



Osmosis> 



