ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 253 



classes. While the molecular weight of most crystalloids 

 is relatively low, that of the colloids is very high. The mo- 

 lecular weight of glue is, for example, about 6000, that of 

 colloidal tungstic acid 1700. 



The following table 1 shows most clearly how a high molec- 

 ular weight and osmotic activity are antagonistic values. 

 The figures refer to 10 percent solutions of the various sub- 

 stances. 



Crystalloids can, moreover, diffuse uninterruptedly through 

 colloidal membranes, such as animal bladders, intestines, 

 sheets of agar-agar, or gelatine, etc. Colloids are for the 

 most part unable to do this. Upon this fact is based the 

 principle of dialysis, in which crystalloids are separated from 

 colloids by placing the mixture in a tube of parchment or 

 an animal bladder, and hanging the whole in water or some 

 other solvent. The crystalloids diffuse out, leaving the 

 colloids behind. 



It must be pointed out at once, however, that no sharp 

 line exists between the colloids, on the one Jiand, and the 

 crystalloids, on the other. Between the two extremes rep- 

 resenting the typical members of these groups there are 

 found an infinite number of substances, which lean more or 

 less strongly toward one side or the other. To illustrate 

 this fact we need only mention that not every crystalloid 

 diffuses through animal membranes, and not every colloid 



HOBER: Zelle und Gewebe. Leipzig, 1902, p. 19. 



