28 DIFFUSION AND OSMOTIC PRESSURE 



the solution and possessing the same temperature. There 

 is, however, a group of substances soluble in water, which 

 do not produce osmotic pressure at all, or produce it in a 

 very slight degree. These are the so-called colloids, such as 

 gelatin, gum, silicic acid, aluminium hydroxid, etc. These 

 substances have very large molecules which diffuse with 

 exceeding slowness and seem to encounter great resistance 

 in passing through water. Colloids in their relation to crys- 

 talloids bid fair to become very important in the advance 

 of physiological knowledge. 



d) Osmotic pressure in general. Most solutes which 

 produce osmotic pressure when in solution are either solids 

 or liquids at ordinary temperatures, when not in solution. 

 But a gas in solution may also produce osmotic pressure if 

 a suitable membrane is employed. 



Osmotic pressure being, in its origin, perfectly comparable 

 to gas pressure, the various principles established for gas 

 pressure have been found to hold for osmotic pressure. The 

 principles of Boyle, of Gay-Lussac, and of Avogadro, devel- 

 oped for gases, have all been extended so as to include sub- 

 stances in solution. Here is convincing evidence that a 

 solute, as long as it is in solution, is essentially a gas occu- 

 pying the volume of the solution. The solvent merely pro- 

 vides conditions under which the pseudo-vaporization which 

 we call solution can take place. Osmotic pressure is inde- 

 pendent of the solvent and is dependent only upon the num- 

 ber of particles of solute (i. e.^ its concentration) and upon 

 their kinetic energy (i. e., their temperature). The nature 

 of the solute is immaterial, the number of particles (mole- 

 cules or ions) per unit volume being, as far as is known, the 

 only essential factor. 



Where several substances that do not react chemically are 

 in very dilute solution, the osmotic pressure of the mixture 

 is the sum of the pressures which would be exhibited were 



