VARIOUS THEORIES OF THE ORGANISM 21 



the more complex the constitution of a substance the more likely 

 it is to exist in the colloid condition. 



The colloids are disperse heterogeneous systems, i.e., they 

 consist essentially of particles larger than molecules of a substance 

 or substances in a medium of dispersion which may be water or 

 some other fluid. In the colloid solution, or "sol," the particles 

 are suspended and separated from each other by the medium, 

 while in the coagulated condition, or "gel," they are more or less 

 aggregated. As regards the size of the particles, the colloid may 

 range from a suspension or emulsion in which the particles are 

 visible to the naked eye to the molecular true solution at the oppo- 

 site extreme. The colloids are usually divided into two groups, 

 the suspensoids, in which the particles are solid, and the emul- 

 soids, in which they are fluid or, more properly, contain a high per- 

 centage of fluid. 



The suspensoids are comparatively unstable as regards the 

 colloid condition, are readily precipitated or coagulated by salts, 

 carry a constant electric charge of definite sign, are not viscous, 

 usually do not swell, do not show a lower surface tension than the 

 pure medium of dispersion, and are mostly only slightly reversible. 



The emulsoids, however, are comparatively stable as colloids, 

 less readily coagulated by salts, may become either positively or 

 negatively charged, are usually viscous and possess a lower surface 

 tension than the medium of dispersion, form membranes at their 

 limiting surfaces, and are reversible to a high degree. 1 



Most of the organic colloids together with some other sub- 

 stances belong to the second group, the emulsoids, and it is demon- 

 strated beyond a doubt that many of the characteristic features of 

 living organisms are due to the presence of a substratum composed 

 of these colloids. The viscosity, the reversible changes in aggre- 

 gate condition through all gradations from sol to gel and back 

 again, the ability to take up water and swell, and the formation of 

 membranes as well as the other properties are of great significance 



1 Books on colloids are rapidly becoming numerous. See for example Freund- 

 lich, '09, and Wolfgang Ostwald, '12, as general works on the subject. Bechhold, 

 '12, Hb'ber, 'n, and Zanger, '08, consider the significance of the colloids for the living 

 organism. 



