512 THE RELATION OF COLLOIDS AND LIPOIDS TO IMMUNITY 



are not diffusible. Colloids occupy a place between the true solutions of 

 crystalloids and the emulsions. Sharp boundaries cannot usually be 

 drawn between any of the members of the series. They differ quanti- 

 tatively in some manner from the true solutions and the emulsions, but 

 may approach them closely, and sometimes resemble them so strongly 

 as to be almost indistinguishable from them. For the most part, 

 however, they show decided characteristics that will differentiate them 

 from the crystalloids, on the one hand, and the suspensions, on the other. 



Those colloids that closely resemble the true solution have been 

 designated " colloidal solutions," and those resembling more closely the 

 suspensions, "colloidal suspensions." Of the two types, the colloidal 

 solutions are far more important biologically, since the colloidal suspen- 

 sions are usually prepared artificially and seldom occur in nature. 



Colloids, therefore, appear to be suspensions of masses of molecules, 

 or perhaps of very large single molecules. When these aggregations 

 are sufficiently large, we have an ordinary suspension. 



1. Colloids are usually amorphous in character, and with few excep- 

 tions do not present a typical structure; they are not crystalline under 

 any visible condition. This, however, is not invariably the case, for 

 we may have a protein, like hemoglobin, which resembles a typical col- 

 loid in every respect, and may yet form crystals readily and abundantly. 



2. Colloids do not form true solutions, but the solvent is probably an 

 important factor in determining whether or not a substance is colloidal 

 in nature; e. g., soaps form true solutions in alcohol and colloidal solu- 

 tions in water; rubber forms colloidal solutions in ether, but not in 

 water. The term colloidal solution does not, therefore, refer to a true 

 solution in the sense of a crystalloid, but to a colloidal state of suspen- 

 sion (the so-called colloidal solution). 



3. Colloids are non-diffusible, or lack the power of passing through 

 animal and parchment membranes. Not all colloids possess the same 

 rate of diffusion, this property being relative, rather than absolute; 

 however, solutions of salts (crystalloids) pass through so readily that 

 they are easily separated from proteins (colloids) by dialyzation, a 

 process that is in constant practical use. 



4. Colloids have an extremely small osomotic pressure. They may, 

 to a very slight degree, exert some influence upon osmotic pressure, the 

 freezing and boiling-points of fluids, but in all cellular processes in which 

 manifestations of osmotic pressure or diffusion are present the crystal- 

 loids may be considered as almost entirely responsible for these. 



5. The colloids exhibit surface tension to a high degree in other words, 



