304 PHYSIOLOGY 



atmosphere, and then as it percolates through the soil and rocks or flows 

 over their surface. Hence, natural waters hold many solutes, and are 

 almost always in position to acquire more if any are removed by chemi- 

 cal action. Thus, the water in arable soils contains everywhere much 

 the same amounts and kinds of mineral salts; for, though soils differ 

 greatly in the proportion of their constituents, the quantities are kept 

 nearly constant by the steady dissociation of the dissolved minerals, 

 by the further solution of any substance which has disappeared from 

 the water for any reason, and by the movement of solutes from one point 

 to another. 



Diffusion. — If solutes are free to diffuse through the water to its utmost 

 limits, what determines the direction and rate of this movement? Im- 

 agine a crystal of a soluble salt placed in a tumbler of water (fig. 621). 

 The particles fly off from the surface and become numerous in the water 

 immediately adjacent. Here, freed partly from the mutual constraint 

 of the crystalline condition, they may be 

 conceived to be in rapid movement to and 

 fro, colliding often with their fellows where 

 these are most numerous and less often where 

 they are fewer. Hence, in regions towards 

 the crystal, rebuffs are most frequent; con- 

 sequently the particles are continually work- 

 ing out into parts of the solvent more and 

 more remote from the crystal and the crowd 

 Fig. 621. — Imaginary sec- of salt particles, the final result being an equal 

 tion of a tumbler of water with jjg^j.j,^mjQjj throughout the Solvent. The 



a soluble crystal, showmg by '^ 



arrows the direction of diffusion, movement is from the region where the par- 

 and by dotted circles the lines of tj^igg ^^^ jji^gj numerous to that where they 



equal concentration. . . -^ 



are less numerous, t.e. from the regions of 

 higher concentration of the solute to regions of lower. Or, since gas pres- 

 sure is conceived to be due to the impact of the molecules on the sides of 

 the container, and since the solute behaves as a gas, it is from regions 

 of higher to regions of lower pressure. For convenience, the ten- 

 dency of solutes to diffuse may be called diffusion pressure or diffusion 

 tension. 



Rate of diffusion. — The rate of movement of diffusing particles of any 

 solute depends on the difference in concentration, or the gradient of the 

 pressure. Thus, when a very soluble crystal is put into a solvent, the 

 rate of diffusion is at first rapid, because an infinitely high concentration 



