42 INTRODUCTION 



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

 particles of a solvent that they remain permanently 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 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 refract or reflect light impinging upon 

 them, exerts a large osmotic pressure, but does not concluct 

 electricity. The other, in which ionization has occurred, differs 

 solely in its capacity to conduct electricity readily. Both are 

 true solutions of crystalloids ; the one which does not ionize is a 

 non-electrolyte ; the other, by virtue of its ionization, is an electro- 

 lyte, 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 mechanical means, can be suspended and uniformly 

 distributed through a fluid without having any marked tendency 

 to aggregate or settle out. Such suspensions or emulsions con- 

 tain particles visible under the microscope, usually appear 

 turbid, refract light, are non-diffusible, exert no osmotic 

 pressure, and do not transmit electricity. Such mixtures are 

 obviously very different from the true solutions above described. 



Between these two extremes stand the colloids, which vary 

 in their properties so that they approach sometimes the suspen- 

 sions (e. g.j lecithin, or coagulated egg-albumin in colloidal sus- 

 pension), 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 ; to illus- 

 trate, arsenic sulphide may appear as a suspension in water, 

 with such large aggregations of its particles that most or all of 

 it can be removed by an ordinary filter. It may exist, how- 

 ever, in the form of a colloidal solution or suspension, which 

 appears perfectly homogeneous to the naked eye, but when 

 placed under the microscope, the fluid is found to be filled with 

 minute particles in Brownian movement. Still other colloidal 

 suspensions of the same substance may be obtained which with 

 the best oil-immersion lenses show no particles of any kind, 

 but when these solutions have a beam of light passed through 



