THE CHEMISTRY AND PHYSICS OF THE CELL 41 



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 slowly 

 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 clas- 

 sified among them, but never are glue-like, e. g., the colloidal 

 metals, so that the name has lost some of its original signifi- 

 cance.) The physical property 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 was 

 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 

 " pectous " 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 pro- 

 teids, and also gums, starch, dextrin, glycogen, tannin, chondrin, 

 perhaps the soaps and lecithin, 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 elements 

 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 primary 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 suspensions or emulsions 

 on the other ? An enormous mass of literature has been piled 

 up by the workers upon the problems here presented, and 

 although the last word has not been said, yet a fair understand- 

 ing of the conditions of solution has been reached, and many 

 important properties have been discovered and explained. The 

 sum and substance of our present conception of the nature of 

 colloidal solution may be briefly summarized as follows : 



