SECTION II 



THE PASSAGE OF WATER AND DISSOLVED 

 SUBSTANCES ACROSS MEMBRANES 



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

 the dissolved substance or solute is not uniform, there is a movement 

 of the substance from the place of higher to the place of lower concen- 

 tration, 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 intermixture of gases is attained. The move- 

 ment in the case of dissolved substances, 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 squaie 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 

 differences 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 infinitesimally 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 

 diffuses in a unit of time from A to B. Thus the statement that the 

 diffusion coefficient of urea is 0*810 at 7*5 C. denotes that if A 

 be continually filled with a 1 per cent, solution of urea, while in B a 

 constant current of distilled water is kept up so as to maintain the 

 concentration at zero, in the course of a day 0*810 gramme of urea 

 will pass from A to B through the cylinder of one centimetre in length 



145 10 



