34 TUB CHEMISTRY AND PHYSICS OF THE CELL 



COLLOIDS 18 



Since Graham in 1861 studied the differences between the sub- 

 stances that did or did not diffuse readily through animal or parch- 

 ment membranes, soluble substances have been classified in the two 

 main groups of colloids and crystalloids, which distinction Graham 

 believed separated two entirely different classes of matter. Although 

 at the present time the differences between the two classes do not 

 seem so great, yet the same division is found useful in classification. 

 By colloids Graham indicated those substances which were dissolved 

 to the extent of showing no visible particles in suspension, but which 

 either did not pass through diffusion membranes at all, or did so very 

 si owl}' indeed, as compared to the crystalloid substances. Under cer- 

 tain conditions they tended to assume a sticky, glue-like nature, 

 hence the name. (Many substances are now known which have the 

 chief properties of the colloids and are therefore classified among 

 them, but never are glue-like, e. g., the colloidal metals, so that the 

 name has lost some of its original significance.) The physical prop- 

 erty which Graham particularly noted in the colloids, besides their 

 non-diffusibility, was the tendency to assume various states of solidity. 

 Not only can they be in solution, when he called them "sols" (when 

 the solvent Avas water, "hydrosols"), but they can become quite firm 

 although containing much water (then called "gels" or "hydrogels"). 

 The gels may assume a firm, coagulated condition, the so-called "pec- 

 tous" state, which state is permanent in that the gel form cannot be 

 reobtained from the pectous modification. Finally the colloid can be 

 in a dry, solid state, quite free from water, and then not a sol at all. 



Included in the great class of colloids are all forms of proteins, 

 and also giims, starch, dextrin, glycogen, tannin, chondrin, probably 

 the enzymes, and also the greater number of organic dyes; also there 

 are inorganic colloids, such as silicic acid, arsenic sulphide, hydrated 

 oxide of iron, and many other similar compounds, besides the ele- 

 ments themselves, especially the noble metals, which may exist in col- 

 loidal form. It will be seen at once that the chief constituents of the 

 cells, in fact nearly all the primaiy constituents except the inorganic 

 salts, are organic colloids, and therefore the properties of the cells 

 are largely dependent upon the properties of the colloids. 



In considering the characteristics of the colloids we at once meet 

 the question — "What distinguishes the colloids from the crystalloids, 

 on the one side, and from susjx'nsions or emulsions on the other? 

 The sum and substance of our present conception of the nature of 

 colloidal solution may be briefly summarized as follows: 



1^ For full disnissinns of tho nature of colloids soo: TTolior. "Physikalisclie 

 Clicinio (Icr Zcllc." Loip/,i<r. 1014 ; Pauli, Kr<:olinisso dcr Pliysiolotrio, 1007 ((>), 

 10;"); r.ocliliold, "Die Kolloidc in Biolo^io und :\1(Hlizin," Drosdon. 1!)12: Wo. Ost- 

 wald, "Crundriss dor Kolloidcliomio,"' Drosdon, 1000; franslafod by 'M. IT. Fischer, 

 101.'). A good brief discussion of colloids is f!;iven by Young in Zinsser's "Infection 

 and Itesistance," 1014. 



