166 



ANALYSIS OF THE ENVIRONMENT 



to the concentration gradient. The natural 

 constant so obtained is called the diflFusion 

 constant. This is Pick's "law," similar to 

 Newton's "law of cooHng," and is one form 

 of the more general "law of velocities." It 

 follows that, as diffusion equilibrium is ap- 

 proached, the rate of diffusion is steadily 

 decreased. 



Many of these statements about diffusion 



eddy coeflBcient in analyzing diffusion in 

 natural environments. The transport result- 

 ing from turbulence may be many times 

 greater and more rapid than that from dif- 

 fusion, and under many conditions the 

 rapidity and completeness of the intermix- 

 ture of diffusible substances are primarily 

 dependent on turbulence rather than on 

 diffusion. 



DIFFUSION GRAofENT ot BEGINNING 



DIFFUSION GRADIENT of TOTAL SOLUTES 

 at EQUILIBRIUM of CONCENTRATION 



DIFFUSION GRADIENT of SOLVENT 

 at EQUILIBRIUM of CONCENTRATION 



Fig. 37. Direction and relative rates of diffusion of solutes and solvent. (Redrawn from 



MiUer.) 



are illustrated in the diagrams shown in 

 Figure 37. For our purpose, the figure is 

 self-explanatory. Diffusion processes are dis- 

 cussed thoroughly for the physiologist in 

 relatively simple, mathematical terms by 

 Jacobs (1935). There is no equally compe- 

 tent monographic presentation of the sub- 

 ject from the ecological point of view. In 

 ecology, turbulence introduces a disturbing 

 relationship in the process of diffusion, and 

 it is often necessary to introduce a so-called 



The rate of diffusion stands apart from 

 many chemical and biological processes in 

 that it is little affected by changes in tem- 

 perature. The velocity of molecular move- 

 ment in gases, and probably in liquids, is 

 proportional to the square root of the abso- 

 lute temperature. If the velocity is 1000 at 

 20° C, it wdll increase to 1017 at 30°. Thus 

 exponential temperature 'laws" (p. 107) 

 do not apply to this process. 



The practical results of the operation of 



