THE COLLOIDAL CONDITION 205 



brane of a simple dialyzer, cannot be filtered out of solution, do not 

 settle out under the action of gravitation, and are visible only 

 under the " ultramicroscope" ; and one which has certain peculiar 

 optical, osmotic, and other physical and chemical properties. 

 Since colloidal particles are very minute in size, they possess very 

 large relative surface areas as compared with their total mass or 

 volume, very high surface tension, and a relatively high surface 

 energy as compared with their total, or molecular, energy. These 

 properties bring into play, in a substance which is in the colloidal 

 condition, in a remarkable degree, all the phenomena which are 

 associated with surface boundaries between solids and liquids, 

 liquids and gases, etc. 



The properties arising out of the colloidal condition are of such 

 tremendous importance in connection with the vital phenomena 

 exhibited by cell protoplasm that it is necessary to give some 

 detailed consideration to them here. Many large volumes dealing 

 with this condition of matter have been written, and it is very dif- 

 ficult to condense even the most important facts concerning it 

 into a few pages, but an attempt has been made to present in this 

 brief summary the most essential facts and principles involved 

 in the colloidal phenomena. 



NOMENCLATURE AND CLASSIFICATION 



Colloidal mixtures may exist in two different forms: one, in 

 which the mixture is fluid and mobile, like a true solution, is known 

 as a " sol"; and the other, which is a semi-solid, or jelly-like, form, 

 is known as a " gel." Sols may be easily converted (or " set ") 

 into gels, by changes of temperature or of the electrolyte content, 

 or by changes in the concentration of the mixture, etc., and in 

 most cases gels can be converted again into sols. In some cases, 

 however, gel-formation is irreversible, the gels are permanent and 

 cannot be changed back again into sols by any known change in 

 environmental conditions. 



Depending upon whether the liquid dispersion medium is 

 water, alcohol, ether, etc., sols are known as " hydrosols," " alco- 

 sols," " ethersols," etc.; and gels as " hydrogels," " alcogels," etc. 



Sols in which the disperse phase is a solid are known as " sus- 

 pensoids"; while those in which it is a liquid are " emulsoids." 

 Thus, sols of most inorganic compounds, of dextrin, gelatin, and 



