COLLOIDS 33 



described. Between these two extremes stand the colloids, which vary- 

 in tlieir properties so that they approach sometimes the suspensions 

 (e. g., lecithin, or coagulated egg-albumin in colloidal suspension), 

 and sometimes more nearly the true solutions (e. g., dextrin). No 

 sharp boundaries can be drawn between any of the members of the 

 series. Indeed, one substance may present all the different stages 

 under different conditions, some agreeing with the properties of the 

 typical suspensions, and some with the properties of the true solutions. 

 The colloids stand in an intermediary position, differing quantitatively 

 in one way or another from the true solutions, but yet approaching 

 them closely and sometimes almost indistinguishably resembling them. 

 For the most part, however, they show characteristics decided enough 

 to entitle them to separate classification, and to make any confusion 

 with the crystalloids impossible. 



The Characteristics of Colloids. — The chief properties of the 

 colloids 'are, then, as follows: - s^ 



Amorphous Form. — This, like almost all other "colloidal properties," is not 

 absolute, for in egg-albumin, hemoglobin, and various globuUns we have proteins 

 which in every respect are typical colloids, yet they form crystals readily and 

 abundantly. Oxyhemoglobin, the molecular weight of which is calculated at 

 about 14,000. exhibits Tyndall'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 solution the individual molecules must have been free and separate, for 

 other\\dse they could scarcely unite in the definite spatial relations necessary to 

 produce crystalline forms. With these few exceptions, however, the colloids do 

 not present any typical structure, and are not crystalUne under anj^ visible condi- 

 tion. But when they are made insoluble by chemical means they may, under 

 certain conditions, produce rather characteristic non-crystalline structures, a 

 matter that ^\•ill be discussed in a subsequent paragraph. 



Solubility. — Although we speak of "colloidal solutions," this terra does not 

 commit us to the theory of the identity of the solution of colloids ^^^th that of 

 crystalloids. We have above stated wliat 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 perhaps of very large single molecules, 

 and a true solution is Likewise a suspension of single molecules or of ions. When 

 the aggregations of molecules are sufficiently large, we have an ordinarj^ sus- 

 pension; but a single protein molecule is as large as a very great number of mole- 

 cules of such substances as sugar (crystalloid); or tannin, C14H10O9 (colloid); or 

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

 solution of a protein did not behave in many particulars Uke a suspension of mo- 

 lecular aggregates of dimensions similar to the dimensions of protein molecules. 

 Nearly all colloidal solutions show Tj^ndall's phenomenon, \yhich demonstrates 

 the existence of particles in saspension large enough to reflect Ught from their sur- 

 faces." 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 di\dded particles of metal, yet they exhibit all the typical phenomena of 

 colloids, passing through many sorts of filters, and even inducing the same hydro- 

 lytic changes as many enzymes. 



" It is highly probable, however, that Tyndall's phenomenon when exhibited 

 by true colloidal solutions (e. g., soluble proteins), depends on the presence of aggre- 

 gates and not properly on the dissolved colloids. (See McClendon and Prender- 

 gast. Jour. Biol. Chem., 1919 (38), 549.) 



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