ISOTONIC SOLUTIONS 149 



Now, since the molecules of water act like those of a gas and are con- 

 tinually moving to and fro, bombarding all the surfaces of their retainer, 

 the molecules of water in A and B will be continually striking all the surfaces 

 of A and B; but since the membrane is permeable to the water molecules, 

 there will be a continual interchange of molecules between A and B. If now, 

 in one side A we place a solution of sodium chloride, still keeping water 

 in B, the membrane being permeable to the sodium chloride, the first thing 

 we should notice would be an increase in the amount of water in A. For- 

 merly it would have been said that " the salt had attracted the water." Now 

 we should say that the salt had a certain osmotic pressure. The salt, how- 

 ever, being able to pass (dialyse) through the membrane,, will do so, and this 

 will continue until the strength of the two salt solutions, and therefore the 

 osmotic pressure on both sides, is equal. 



Osmotic Pressure. If now in A we place a solution of some soluble 

 colloidal substance to which the membrane is impermeable, or else replace 

 the membrane, M, we used in our former experiment by one which is not 

 permeable to the sodium chloride, and arrange our jar as in figure 125, so 

 as to be able to read off any increase of water which may pass into A, \ve 

 will notice that the amount of liquid in A will continue to increase up to a 

 certain point. Once that point is reached, there will be no further change, 

 since the substance in solution, in A, cannot pass through the membrane as 

 in the previous example. This pressure can be measured and expressed in 

 millimeters of mercury. It is constant for all solutions of this substance 

 that are of the same concentration when measured under like conditions of 

 temperature and pressure, and is called the osmotic pressure of this solution. 



Of the numerous explanations regarding the nature of osmotic pressure 

 which have been more or less satisfactory, a simple one, and one that can 

 be easily understood, is as follows: In figure 125 one surface of the mem- 

 brane is being bombarded by the molecules of a non-diffusible substance 

 mixed with those of a diffusible one (water) ; while the other surface is being 

 bombarded entirely by water molecules. The former condition permits 

 only a fraction of the molecules to diffuse out, since fewer water molecules 

 get to the surface of the membrane; while the latter permits all of the 

 molecules which reach it to pass through. 



Osmotic pressure can be estimated in several different ways in addition 

 to the above, viz., the determination of the freezing-point of the solution, 

 determination of the boiling-point, determination of the electrical conduc- 

 tivity. The results obtained with the various methods agree very closely. 

 The following solutions have the same osmotic pressure: Sodium chloride, 

 i . 64 per cent. ; potassium nitrate, i . 09 per cent. ; sugar 5 . 5 per cent. 



Isotonic Solutions. Solutions that have the same osmotic pressure 

 are called isotonic. The term isotonic is a relative one, implying a compari- 

 son with some other solution taken as a standard. In physiology it has been 



