36 THE CHEMISTRY AHiD PHYSICS OF THE CELL 



dall's phenomenon and will not pass through a very fine porcelain 

 filter, and therefore resembles the colloids decidedly, yet it forms 

 beautiful crystals. The very fact that crystals are formed, Spiro 

 points out, is proof that when in soluti(jn tlie individual molecules 

 must have been free and separate, for otherwise they could scarcely 

 unite in the definite spatial relations necessary to produce crystalline 

 foi-ms. With these few exceptions, however, the colloids do not pre- 

 sent 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 sub- 

 sequent ])aragTaph. 



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 w^hat seems 

 to be a fair view of the matter as shown by many methods of experi- 

 mentation. Most colloids seem to be, in fact, suspensions of masses 

 of molecules, or perhaps of very large sing:le molecules, and a true 

 solution is likewise a suspension of single molecules or of ions. When 

 the aggregations of molecules are sufficiently large, w^e have an ordi- 

 nary suspension ; but a single protein molecule is as large as a very 

 great number of molecules of such substances as sugar (crystalloid) ; 

 or tannin, C14H10O9 (colloid) ; or calcium carbonate (insoluble, sus- 

 pension) ; and it would be strange if a true solution of a protein 

 did not behave in many particulars like a suspension of molecular 

 aggregates of dimensions similar to the dimensions of protein mole- 

 cules. Nearly all colloidal solutions show Tyndall's phenomenon, 

 which demonstrates 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 al- 

 most entirely retained by a hardened paper filter, while others pass 

 through the finest-pored clay filters. Furthermore, the metallic col- 

 loids, 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 filtei-s, 

 and even accomplishing the same hydrolytic changes as many en- 

 zymes. 



It must also be mentioned that the solvent is probably an im- 

 portant factor in determining tlie colloidal or noii-colloidal nature 

 of a substance ; e. g., soaps form true solutions in alcoliol and colloidal 

 solutions in water; gelatin forms colloidal solutions in water but not 

 in ether, whereas rubber forms colloidal solutions in ether but not in 

 water. 



Closely I'elati'd to solubility is tiie ])li('ii(itiieii()n ol' iuihihittnn (the 

 "Quellung" of German writers), which inny he deliuiMl ;is the tak- 



