32 INTRODUCTION TO GENERAL PHYSIOLOGY 



"colloidal," of which the dispersed phase is in large enough 

 particles, be these aggregates or single molecules, not to pass 

 through parchment paper, while at the same time small enough 

 to remain suspended permanently, or for a long time. 



When such solutions were first described by Thomas Graham, 

 the colloidal state was thought to be a property of certain sub- 

 stances, such as gelatin or glue only ; hence the name (xoAA.?/, glue). 

 But we now know that any substance, by appropriate treatment, 

 can be brought into the state. In general, the treatment may be 

 described as reducing the material in question by some means or 

 other to a very fine state of subdivision. In the case of chemical 

 elements or simple compounds of small molecular dimensions, the 

 colloidal particles are aggregates of a large number of separate 

 molecules, but it is obvious that a single molecule, if large enough, 

 may exhibit colloidal properties. Such is the case with some dyes, 

 as congo red, and with the proteins, of great physiological im- 

 portance, whose nature we shall learn in the next chapter. We 

 must remember that the visibility of the particles depends on the 

 brilliancy of the illumination, and on the fact whether they differ 

 much in refractive power from the liquid in which they float. 

 There are, indeed, some substances which we know to be in 

 colloidal solution, because they do not pass a parchment paper 

 membrane, but which require a very powerful illumination to show 

 the presence of particles. Some only show a diffuse beam of light 

 when observed under the best conditions yet possible ; they have 

 not been actually resolved into separate particles. On the other 

 hand, if the illumination is sufficiently powerful, even simple 

 molecules may show a beam of scattered light ; in fact the blue 

 of the sky is such light scattered by the molecules of the gases 

 of the atmosphere. 



We must now direct some attention to the properties which 

 belong to colloids in consequence of their enormous development 

 of surface. First of all; there are certain properties due to the 

 presence of surface tension, or rather of surface energy. Since the 

 larger the number of particles into which a given mass is divided, 

 the greater the total area of surface, there will always be a tendency 

 for these particles to aggregate together again into larger masses, 

 for by doing so there wfll be a diminution of free surface energy. 

 This tendency is opposed by the continual Brownian movement, 

 and we Tan also decrease it by diminishing the intensity factor 

 of surface energy, that is, the surface tension, by the addition 

 of some substance which lowers the surface tension at the interface. 

 Solutes in general do this, as we have seen, but there are some 

 which have a very marked effect of this kind. Such are the higher 

 alcohols, bile salts, fatty substances, saponin, etc. (E., p. 177). But 



