COLLOIDS 35 



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

 ticles of a solvent that they remain permanently in this condition, 

 neither aggregating into masses nor separating out through the action 

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

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

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

 other substances, as salt, go still further, and the molecule divides into 

 two or more parts, which have different electric charges (ionization) . 

 The first of these classes of substances forms a solution which con- 

 tains no particles visible by any known means, does not contain 

 particles large enough to reflect light impinging 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 electricity readily. Both are true solutions of crj-stal- 

 loids; the one which does not ionize is a non-electrolijte ; the other, by 

 virtue of its ionization, is an electrolyte, the ions carrying 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 be suspended and uniformly distributed through 

 a fluid without having any marked tendency to aggregate or settle 

 out. Such suspensions or emulsions contain particles visible under 

 the microscope, usually appear turbid, refract light, are non-diftusible, 

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

 tures are obviously very different from the true solutions above de- 

 scribed. Between these two extremes stand the colloids, which vary 

 in their properties so that they approach sometimes the suspensions 

 (e. g., lecitliin, 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 tlie 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 quanti- 

 tatively in one way or another from the true solutions, but yet ap- 

 proaching them closely and sometimes almost indistinguishably re- 

 sembling them. For the most part, however, they show character- 

 istics 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 : 



Amorphous Fcrai. — This, like almost all other "colloidal properties," 

 is not absolute, for in egg-albumin, hemoglobin, and various globulins 

 w^e have proteins which in every respect are typical colloids, yet 

 they form crystals readily and abundantly. Oxyhemoglobin, the mo- 

 lecular weight of which is calculated at about 14,000, exhibits Tyn- 



