PHYSICOCHEMICAL AND CHEMICAL PHASES 



165 



It is difficult to distinguish clearly and 

 accurately between modern physics and 

 chemistry. The two meet in the annectent 

 science of physical chemistry. Physicochemi- 

 cal aspects of ecology include such phenom- 

 ena as diffusion, osmosis, hydrogen ion 

 concentration (acidity and alkalinity), 

 chemical buflFering of the enyironment, 

 other ion effects, and adsorption. Solute re- 

 lations in aqueous solutions, including the 

 unique colligatiye properties associated with 

 osmotic pressure— the lowering of the freez- 

 ing point, the lowering of yapor pressure, 

 the eleyation of the boiling point— and sur- 

 face tension, have been or will be consid- 

 ered in part in connection with other topics 

 rather than directly (see Index). Ecological 

 applications of colloidal chemistry, despite 

 probable importance, haye been slightly 

 deyeloped and will be discussed only 

 briefly. All these are, in the main, physico- 

 chemical characteristics of the enyironment, 

 but no attempt will be made to distinguish 

 between physicochemical and more strictly 

 chemical reactions within the nonhying en- 

 yironment. Neither will the line be drawn 

 sharply between possibly biotic efiFects and 

 those that result from reactions between 

 nonhying systems. 



VISCOSITY 



Viscosity of water results from cohesion 

 among the water particles, including water 

 molecules. Under appropriate conditions, 

 adhesion to rocks, sand, mud, or other con- 

 stituents of the shores or bottom aflFect the 

 expressed yiscosity. The greater the vis- 

 cosity, the more resistance is oflFered to 

 changes in form and to moyement. Under 

 ideal conditions, a streamlined body moving 

 at an appropriate speed parts the water 

 without ripples or eddies and initiates a 

 series of layers gliding smoothly past each 

 other. This provides an example of laminar 

 yiscosity such as is rarely realized in nature. 

 Rather, turbulence is produced, at least to 

 some extent, and the resulting confused or 

 smoothly developed system of vortices pro- 

 vides examples of eddy viscosity that is at 

 once much more complex and many times 

 greater than laminar viscosity. The intrinsic 

 difficulty in analysis of eddv viscosity in 

 precise terms is indicated by the mathemat- 

 ical discussion in Sverdnip, Johnson, and 

 Fleming (1942, p. 469). 



Viscosity of water is related to cyclomor 

 phoses (p. 118), to flotation in general (p. 



132), as well as to the streamlined form of 

 many aquatic animals (p. 156). Viscosity 

 increases decidedly with lowered tempera- 

 ture, increases somewhat with sabnity, but 

 is only slightly affected by pressure even in 

 the depths of the ocean (p. 137). The gen- 

 eral ecological relations of aquatic animals 

 to the yiscosity of water are simply and cor- 

 rectly outhned by Coker (1947). Similar 

 relations with air hold for land animals, ex- 

 cept that the viscosity values are less. 



DIFFUSION 



In aqueous solution and in natural mix- 

 tures of gases of ecological importance, as 

 well as in solutions and gases less directly 

 related to ecology, all the molecules or ions 

 present move more or less freely through 

 the whole. The movement is free with 

 gases, less so in liquid solutions, still less 

 free in those solutions that approach solids, 

 and least free when the solvent is a solid. 

 Many essential materials in the environment 

 of organisms owe their tendency toward 

 uniform distribution to diflfusion, and dif- 

 fusion through membranes, called osmosis, 

 places organisms in effective working rela- 

 tions with many aspects of their environ- 

 ment. 



The relatively simple but important facts 

 concerning diffusion, uncomplicated by con- 

 siderations of membrane permeability, may 

 be summarized both for solutions and for 

 gaseous mixtures as follows: All the ions 

 and molecules present as solvent, or dis- 

 solved solute, tend to diffuse throughou^ 

 the whole available space; such diffusion is 

 active and continuous. The diffusion of each 

 kind of ion or molecule is almost independ- 

 ent of other kinds that may be present; 

 thus, within the limits of normal sea water, 

 the rate of diffusion is almost independent 

 of salinity. 



While ions or molecules move in random 

 fashion, collision with other similar particles 

 is more frequent toward the region of their 

 greater concentration; hence there is a tend- 

 ency for net movement to be toward the 

 region of greater dilution of the given ion 

 or molecule regardless of the position of 

 greatest concentration of the sum total of 

 all substances present. This principle ap- 

 plies both to the concentrations of the sol- 

 vent and of the solute. 



The rate of diffusion across any plane at 

 right angles to the direction of diffusion 

 bears a simple, linear, quantitative relation 



