THE NEW THEORY OF SOLUTIONS. 13 



to the membrane employed. The magnitudes of this 

 quantity for different dissolved substances have no very 

 precise meaning, however, for besides being affected by 

 the pressure differences which are set up upon the mem- 

 brane during endosmose they are largely influenced by 

 the nature of the membrane. It appears that the kind 

 of membrane used, or with the same membrane, its 

 thickness or freshness modifies the results obtained. 

 Even the direction of endosmose is of importance in 

 some cases : thus water passes more readily outwards 

 through eel's skin, more readily inwards through frog's 

 skin. 



OSMOSE. 



For these reasons it is evident that when animal 

 bladder is employed the behaviour of dissolved sub- 

 stances towards permeable partitions is to a large extent 

 masked by the specific effect exerted by the membrane. 

 In more recent times this disturbing influence has 

 apparently been entirely removed and the phenomena 

 have been considerably simplified by the use of mem- 

 branes artificially prepared. The credit of directing 

 attention to the properties of these artificial membranes 

 is due to M. Traube. Working still from a physiological 

 stand-point he discovered, in 1867, that several mem- 

 branous precipitates although they may be freely traversed 

 by water differ from animal membranes in the important 

 particular of presenting a barrier to the passage of 

 certain dissolved substances. Traube showed, for in- 

 stance, that if a quantity of copper sulphate solution be 

 aspirated into a narrow glass tube, on cautiously dipping 

 the tube into a solution of potassium ferrocyanide, 

 and allowing the two solutions to come into contact, a 

 membranous precipitate of copper ferrocyanide may be 

 formed, shutting in the contents of the tube. This 

 membrane was found to be impermeable to the dissolved 

 substances which produced it and also to certain other 

 substances which could be added to the original solutions. 

 Such a membrane, which is permeable only to sojvent 



