32 THE CHEMISTRY AND PHYSICS OF THE CELL 



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

 the solvent was water, " hydrosols ") , but they can become quite firfn 

 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 gel at all. 



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

 and also gums, starch, dextrin, glycogen, tannin, probably the en- 

 zymes, and also the greater number of organic dyes; also there are in- 

 organic 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 colloidal 

 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? 

 The sum and substance of our present conception of the natiu-e of 

 colloidal solution may be brieflj'' sunmiarized as follows: 



It is possible for solid substances to be so divided among the par- 

 ticles of a solvent that they remain permanentlj^ in this condition, • 

 neither aggregating into masses nor separating out through the action 

 of gravity. With some substances, as sugar, for example, the solid 

 seems to divide up into its molecular form, each molecule being free from 

 all others of its kind except during occasional contacts. Some other 

 substances, as salt, go still further, and the molecule cUvides into two 

 or more parts, which have different electric charges {ionization). The 

 first of these classes of substances forms a solution which contains no 

 particles visible by any known means, does not contain particles large 

 enough to reflect light imioingiiig upon them, exerts a large osmotic 

 pressure, but does not conduct electricity. The other, in which 

 ionization has occurred, differs solely in its capacity to conduct elec- 

 tricity readily. Both are true solutions of crystalloids; the one which 

 does not ionize is a n.on-electroh/tc; the other, by virtue of its ionization, 

 is an electrolyte, the ions carrj'ing electric charges through the solution. 



At the other end of the scale we have substances which are quite 

 insoluble when in masses, but which, when very finely divided by me- 

 chanical means, can l)e sus])ended and uniformly distributiul through 

 a fluid without having any maiked tendency to aggregate or settles 

 out. Such suspensions or emulsions contain particles visible under 

 the microscope, usually appe;ii Imhid, refract light, are non-diffusible, 

 exert no osmotic pressure, and do not transmit electricity. .Such 

 mixtur(!s are obviously veiy dilTca'cnt from the true solutions above 



