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 distribudon throughout the solvent. The 
a soluble crystal, showing by 
arrows the direction of diffusion, movement is from the region where the par- 
and by dotted circles the lines of ticles afe most numerous to t hat where they 
equal concentration. . J 
are less numerous, i.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 
