SECTION II 



THE PASSAGE OF WATER AND DISSOLVED 

 SUBSTANCES ACROSS MEMBRANES 



WE have already seen that if, in a solution, the concentration of the dis- 

 solved substance or solute is not uniform, there is a movement of the sub- 

 stance from the place of higher to the place of lower concentration, and 

 this movement proceeds until the concentration is equal throughout the 

 fluid. This movement of dissolved substances through a fluid is spoken of as 

 diffusion, and is analogous in all respects with the process by which the inter- 

 mixture of gases is attained. The movement in the case of dissolved sub- 

 stances, as of gases, takes place from the region of higher to the region of lower 

 (osmotic) pressure. It can therefore be ascribed to differences of pressure, 

 or rather to the factor which we regarded as responsible for the production of 

 the pressure, viz. the movement of the molecules themselves. The rate of 

 diffusion is not the same for all substances. In gases the rate varies inversely 

 as the square root of the density of the gas. Thus hydrogen (density = 1) 

 diffuses four times as rapidly as oxygen (density = 16). We find similar 

 differences between the rates of diffusion of dissolved substances differ- 

 ences which also are determined in all probability by the weight and 

 size of the individual molecules, although the relation between molecular 

 weight and rate of diffusion is not so simple as the ratio between these two 

 quantities in gases. The diffusibility of a substance is given by its diffusion 

 coefficient. The amount of dissolved substance, which diffuses in a unit 

 of time across a given area of fluid, is proportional to the difference between 

 the osmotic pressures at two cross-sections of the column of fluid at an in- 

 finitesimally small distance apart. If we take a cylindrical mass of solution 

 which is one centimetre long and has a sectional area of one square centimetre 

 (Fig. 23), and maintain a constant 

 difference of concentration between A 

 and B = 1, the diffusion coefficient 

 is the amount of substance which f) 

 diffuses in a unit of time from A to 



B. Thus the statement that the 



diffusion co-efficient of urea is 0-810 



at 7-5 C. denotes that if A be con- FIG. 23. 



tinually filled with a 1 per cent. 



solution of urea, while in B a constant current of dis 

 is kept up so as to maintain the concentration at zero, in the cou: 



131 



