THE COLLOIDAL CONDITION 211 



ulation and peptization cannot be repeated, that is, the process 

 is not a definitely reversible one. 



Importance of Gel-formation. From the physiological point 

 of view, gel-formation is undoubtedly the most important aspect 

 of colloidal phenomena. In the first place, the ability to absorb 

 and hold as much as 80 to DO per cent of water in a semi-solid 

 structure is of immense physiological importance. In no other 

 condition can so large a proportion of water, with its consequent 

 effect upon chemical reactivity, be held in a structural, or semi- 

 solid, mass. But a vastly more significant feature of the condi- 

 tions supplied by the gel lies in the fact that the non-water phase, 

 or phases, of the system are spread out in a thin film, or mem- 

 brane, thus giving it enormous surface as compared with its 

 total volume. This effect is easily apparent if one thinks of the 

 enormous surface which is exposed when a tiny portion of colloidal 

 soap is blown out into a " soap-bubble " several inches in diam- 

 eter. This condition brings into play all the phenomena resulting 

 from surface boundaries between solids and liquids, liquids and 

 liquids, liquids and gases, etc., from surface tension, surface 

 energy, etc. Among these effects may be cited those of adsorp- 

 tion, increased chemical reactivity due to enlarged areas of contact, 

 permeability and diffusion, etc., the importance of which in the 

 vital phenomena of cell-protoplasm will be discussed in detail 

 in the following chapter, 



GENERAL PROPERTIES OF COLLOIDAL SOLUTIONS 



Non-diffusibility. The most characteristic property of all 

 sols is the failure of the suspended particles to pass through a 

 parchment, or any similar dialyzing membrane. 



Visibility under the " Ultramicroscope." The particles of 

 a sol, in contrast with the molecules of a true solution, are visible 

 as bright scintillating points under the ultramicroscope. This 

 is a modification of the type of dark-field illumination of the 

 ordinary microscope, as applied to microscopic studies, in which 

 the solution to be studied is contained in a small tube or box of 

 clear glass which is mounted on the stage of an ordinary micro- 

 scope and instead of being illuminated from below by transmitted 

 light is illuminated by focusing upon it the image of the sun, 

 or of some other brilliant source of light such as an electric arc, 



