544 THE RELATION OF COLLOIDS AND LIPOIDS TO IMMUNITY 



never in equilibrium; conditions which determine the appearance of a 

 body in the colloid or crystalline form appear to indicate that bodies 

 always separate from solution in the amorphous or colloidal condition 

 and that all crystallization is a secondary phenomenon. 



On the other hand, we may have substances that are quite insoluble 

 when aggregated in masses, but when derived in pure form by me- 

 chanical means can be suspended and uniformly distributed through a 

 fluid without showing any marked tendencjr to precipitate. Such 

 suspensions or emulsions contain particles that are visible under the 

 microscope; they usually appear turbid, do not transmit electricity, and 

 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. 



When colloids occur in a highly dispersed state they are called sols; 

 when in an undispersed or but slightly dispersed state they are spoken 

 of as gels. A colloid substance may be converted from a sol state into 

 a gel state and back again, when it is called a reversible colloid or emulsion; 

 a colloid which refuses to redisperse is an irreversible colloid or sus- 

 pension. 



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- 



