PROPERTIES OF MATTER 43 



stood but which is apparently beyond the physiological capacity of 

 human beings from low altitudes. 



In a similar way carbon dioxide deficiencies may be demonstrated 

 for certain microorganisms. In these cases a low pressure of carbon 

 dioxide on the medium leads to too low a concentration in solution, 

 and the cells fail to multiply and may be fatally affected. Such consid- 

 erations apply to all gases involved in biological exchanges, even those 

 given off, for an exceptionally high pressure of a waste gas can lead 

 to toxic concentrations. Furthermore, sudden large decreases in pres- 

 sure sometimes allow dissolved gas to form bubbles causing disturb- 

 ances known as the bends, one of the principal hazards of deep tliving. 



COLLOIDAL SYSTEMS 



When a solution of sodium chloride is placed in a collodion bag 

 and the bag immersed in water, the salt dialyzes rapidly through the 

 collodion film and appears in the water in increasing concentration. 

 However, if starch is suspended in water, none passes through a collo- 

 dion membrane. Presumably this difference in behavior depends 

 upon differences in size. Generally speaking, ordinary solutions in- 

 volve solute particles with diameters of less than 1 to 2 millimicrons, 

 1 to 2 X 10-^ mm. Dispersions of particles exceeding about 200 milli- 

 microns are known as coarse dispersions. The intermediate region 

 includes the colloidal systems with particles of intermediate size. Both 

 size limits are quite arbitrary, and the properties of each group merge 

 gradually into those of the neighboring groups. 



Among the colloidal solutions there are many similar to the ordinary 

 or true solutions in that the dispersed phase is a homogeneous molec- 

 ular dispersion. Suspensions of soluble proteins and large carbohy- 

 drates fall into this category. On the other hand, dispersed aggregates 

 of soaps, clays, and the like are also colloids. Hence the classification 

 is merely one of size and does not correspond closely to groupings of 

 chemical or other physical properties. However, colloids are par- 

 ticularly important in certain respects discussed below. 



Formation 



Those colloids whose individual molecules place them in this size 

 range are obviously the result of natural and biological syntheses. 

 Many of these processes have not yet yielded to the chemist, but others 

 have been duplicated in the laboratory, for example, the production 

 of rubber, plastics, and polymers. In these materials the principal 



I 



