168 PRINCIPLES OF GENERAL PHYSIOLOGY 



According to the kinetic theory, substances in solution must diffuse from 

 places of higher concentration to those of lower concentration, until the same 

 concentration is attained in both. Unlike gases, however, the process is extremely 

 slow, owing to the great resistance met with. 



Since the kinetic energy of a molecule, an ion, or a colloidal particle is the 

 " same, these elements are mutually equivalent as regards the production of osmotic 

 pressure, which depends only on the molar concentration of the elements active. 

 Colloidal solutions, therefore, must possess a true osmotic pressure, which is 

 usually small, on account of the low molar concentration of such solutions in 

 active elements. 



Diffusible impurities play no part in the osmotic pressure of colloids, except 

 in so far as they may affect the degree of dispersion of the particles of the 

 colloidal state. 



The osmotic pressure of electrolytically dissociated salts, of which one ion only 

 is colloidal, requires special consideration. It is shown that the diffusible ions, 

 .although the membrane is permeable to them, play their full part in the production 

 of osmotic pressure. 



Certain special phenomena, of which explanation is given in the text, are 

 present in such cases. A salt of which both ions are diffusible through the 

 membrane, if added to the system, is found, when equilibrium is attained, to have 

 distributed itself in such a way as to. have a lower concentration in the presence 

 of the colloidal salt. This happens whether the two salts have an ion in common 

 or not. There is also a considerable potential difference between the two sides of 

 the membrane, owing to the presence of a permanent " electrical double layer " in 

 that situation. In fact, the system is precisely analogous to a metallic electrode 

 in a solution of one of its salts and the amount of the potential difference is found 

 to be expressed by a similar formula, in which the concentration of the diffusible 

 ions inside the membrane takes the place of the "electrolytic solution tension " of 

 the metal in the formula of Nernst. 



Osmotic pressure, as such, plays a part in various physiological phenomena. 

 The volume of animal cells, the turgor of vegetable cells, the reaction of smooth 

 muscle to drugs, the rate of intracellular reactions, the process of secretion, root 

 pressure, the rate of blood flow, the production of lymph, and the absorption of 

 liquid from tissue spaces are discussed briefly in the text. 



Certain cells possess the power of regulating the osmotic pressure of their 

 contents, while the higher animals have developed mechanisms for maintaining 

 that of their blood and body fluids at a constant value. 



LITERATURE 



General Theory. 



Nernst (1911, pp. 125-161). Van't Hoff (1885). Findlay (1913). 



Otto Stern (1913). Raoult (1900 and 1901). 



Van der Waals* Equation of State. 

 Nernst (1911, pp. 209221). 



Methods. 

 Direct. 



Berkeley and Hartley (1906, 1). Morse (1914). Moore and Roaf (1907). 



Bay liss( 1909, 1). 



Vapour Pressure. 



Barger (1904). 

 Freezing Point. 



Findlay (1906, pp. 110-116). Raoult (1900 and 1901). 



Lymph Production. 



Starling (1909, Lectures IV. and V.). 



