GENERAL STRUCTURE AND COMPOSITION 19 



particles of sand might be filtered and the sand removed, but the 

 solution would have just as much salt dissolved in it after filtra- 

 tion as before. 



OSMOSIS. If we should take such a membranous bag as de- 

 scribed above filled with salt solution and dip it into a vessel of 

 pure water, so that the surfaces within and without the bag are 

 at the same level, it would be seen after a while that the level of 

 liquid within the bag had risen while that in the vessel outside 

 had correspondingly fallen. That is, there would have been an 

 actual movement of water into the bag with sufficient force to 

 overcome the pressure due to gravity resulting from the change of 

 water level on the two sides of the membrane. Whenever two solu- 

 tions of different concentrations are separated by a membrane which 

 is permeable to water there will be a flow of water through the membrane 

 in the direction of the greater concentration. This phenomenon is 

 known as osmosis. The force which drives the water is called 

 osmotic pressure and is said to be exerted by any solution of higher 

 concentration toward any of lower concentration. 



DIALYSIS. A membrane which is permeable to water but not 

 to any particles which may be dissolved in it is known as a semi- 

 permeable membrane; one which allows dissolved substances as 

 well as water to pass is a permeable membrane. When two so- 

 lutions of different concentration are separated by a membrane 

 of this latter class we have in addition to the movement of water 

 under the driving force of osmotic pressure a movement of dis- 

 solved particles through the membrane. This is a special case of 

 the general phenomenon of diffusion. Diffusion may be defined 

 as the tendency of substances in solution to distribute themselves 

 evenly throughout the solvent. Where this distribution necessi- 

 tates the passage of particles through permeable membranes the 

 phenomenon is called dialysis. The effect of both osmosis and 

 dialysis is to equalize the concentrations of the solutions on the 

 two sides of the membrane, but it must be remembered that 

 they are entirely distinct phenomena. To illustrate: suppose we 

 have on the two sides of a permeable membrane solutions re- 

 spectively of sugar and salt of the same concentration, that is, 

 having the same number of particles in solution ; there would then 

 be no flow of water in either direction since the osmotic pressure 

 of both solutions is the same, but since neither the sugar nor the 



