50 UNEQUAL ATTRACTION OF MEMBRANES 



contact with the air, receives least, and is the most 

 highly charged with salt. 



The cause of mixture is the chemical affinity of 

 the salt for the newly-added particles of water ; this 

 affinity is equal on both sides, but the attraction of 

 the substance of the bladder is stronger for the 

 more aqueous or less saline liquid, than for the 

 more concentrated. In consequence of this differ- 

 ence in the attraction of the liquids for the sub- 

 stance of the bladder, a part of the mixture is dis- 

 placed from the bladder ; the less saline liquid 

 takes the place of the more saline ; a part of the 

 latter is expelled, and, with it, a part of that water 

 which has been added to the outer stratum by mix- 

 ture. Brine and water flow out in the direction of 

 least resistance. The efflux towards the side on 

 which the pure water was poured is prevented by 

 the stronger attraction of the more watery liquid 

 for the substance of the bladder. 



If we remove from the outer surface of the 

 bladder the displaced saline liquid (which has been 

 mixed with some water), and put stronger brine in 

 its place, and if on the opposite side we remove the 

 very diluted solution, replacing it by a still more 

 diluted one, the same process is repeated. There 

 arises a permanent difference, and a state of mix- 

 ture and efflux continues till the liquids on the 

 opposite surfaces of the bladder have the same, or 

 very nearly the same, composition. 



If we suppose, that the two liquids moisten the 



