252 PHYSIOLOGY OF ALIMENTATION. 



and since ordinary glue is an example of such he called the 

 substances belonging to this class colloids. On the other 

 hand, those which diffuse rapidly he called crystalloids, for the 

 bodies belonging in this class are mostly crystalline, as sugar 

 and salt. From the physiological standpoint of absorption 

 this difference in the rates of diffusion still stands as one of 

 the most important differences between these two classes of 

 compounds, for, as we shall see, our food is made up to a 

 large extent of typical representatives of these classes. 



Modern advances in physical chemistry have given us 

 other criteria besides differences in the rates of diffusion and 

 in their amorphous or crystalline constitution by which we 

 can distinguish between colloids and crystalloids. When 

 dissolved in water or other solvents the colloids do not form 

 true solutions, but remain suspended in the liquid. Colloidal 

 solutions are, therefore, heterogeneous. More correctly put, 

 a colloidal solution represents a mixture of two substances 

 which are only partially soluble in each other. We formerly 

 looked upon crystalloidal solutions as homogeneous. Re- 

 cent experiments indicate, however, that these too are hetero- 

 geneous, only much less markedly so than the colloidal solu- 

 tions. 



Solutions of crystalloids show an osmotic pressure which 

 is proportional to the number of particles of dissolved sub- 

 stance contained in the unit volume of the solvent. As 

 will be seen later it is upon this fact as well as upon the minute- 

 ness of the dissolved particles that the great diffusibility of 

 the crystalloids depends. In contrast herewith the so-called 

 " typical " colloids show no osmotic pressure and in conse-^ 

 quence do not diffuse at all. But only very few "typical" 

 colloids exist; the vast majority show some osmotic pres- 

 sure and some diffusibility, even though it be but 

 slight. 



These enormous differences in osmotic pressure between 

 crystalloids and colloids correspond to similar differences in 

 the molecular weight of the substances composing the two 



