44 INTRODUCTION 



Graham's term of " crystalloid/' therefore, does not strictly 

 express the distinction he intended, or, rather, the distinction he 

 intended does not exist in so decided a way as he imagined. 

 With these few exceptions, however, the colloids do not present 

 any typical structure, and are not crystalline under any visible 

 condition. But when they are made insoluble by chemical 

 means they may, under certain conditions, produce rather 

 characteristic non-crystalline structures, a matter that will be 

 discussed in a subsequent paragraph. 



Solubility. Although we speak of " colloidal solutions, " 

 this term does not commit us to the theory of the identity of 

 the solution of colloids with that of crystalloids. We have 

 above stated what seems to be a fair view of the matter as 

 shown by many methods of experimentation. Most colloids 

 seem to be, in fact, suspensions of masses of molecules, or per- 

 haps of very large single molecules, and a true solution is like- 

 wise a suspension of single molecules or of ions. When the 

 aggregations of molecules are sufficiently large, we have an 

 ordinary suspension ; but a single proteid molecule is as large 

 as a very great number of molecules of such substances as 

 sugar (crystalloid) ; or tannin, C 14 H 10 O 9 (colloid) ; or calcium 

 carbonate (insoluble, suspension) ; and it would be strange if a 

 true solution of a proteid did not behave in many particulars 

 like a suspension of molecular aggregates of dimensions simi- 

 lar to the dimensions of proteid molecules. Nearly all col- 

 loidal solutions show TyndalPs phenomenon, which demon- 

 strates the existence of particles in suspension large enough to 

 reflect light from their surfaces. Most of the colloids are held 

 back by very fine filters to a greater or less degree ; some are 

 almost entirely retained by a hardened paper filter, while others 

 pass through the finest-pored clay filters. Furthermore, the 

 metallic colloids, such as those of platinum, gold, and silver, 

 are unquestionably suspensions of finely divided particles of 

 metal, yet they exhibit all the typical phenomena of colloids, 

 passing through many sorts of filters, and even accomplishing 

 the same hydrolytic changes as many enzymes. 



It must also be mentioned that the solvent is probably an 

 important factor in determining the colloidal or non-colloidal 

 nature of a substance ; e. g., soaps form true solutions in alco- 

 hol and colloidal solutions in water ; gelatin forms colloidal 

 solutions in water but not in ether, whereas rubber forms col- 

 loidal solutions in ether but not in water. 



Closely related to solubility is the phenomenon of imbibition 

 (the " Quellung " of German writers), which may be defined as 



