THE COLLOIDAL CONDITION 213 



to the illumination of the dust particles in the air of a darkened 

 room when a ray or narrow beam of light passes through it. In a 

 true molecular solution, the particles are too small to be visible by 

 this mode of illumination. 



Other Optical Properties. Sols are generally translucent and 

 opalescent; many of them are highly colored, some of the sols of 

 gold, platinum and other heavy metals possessing particularly 

 brilliant colors. In general, metallic suspensoids are red, violet, 

 or some other brilliant color; while inorganic suspensoids are bluish 

 white, and emulsoids generally blue to bluish white. 



Formation of Froth, or Foam. Colloidal solutions, especially 

 those of the natural proteins, fats, glucosides, gums, and the 

 artificial soaps, have a strong tendency to produce froth, or 

 foam, when shaken; this being due to the enormous surface ten- 

 sion resulting from the finely divided condition of the dispersed 

 material. 



Low Osmotic Pressure. All colloidal solutions exhibit a very 

 low osmotic pressure; the freezing point of the dispersion medium 

 is lowered only very slightly and its boiling point is only very 

 slightly raised by the presence of the dispersed particles in it. 



Precipitation by Electrolytes. Sols of all kinds are precip- 

 itated, or caused to form gels, by the addition of electrolytes^ 

 since these cause a disturbance of the electric charge on the 

 dispersed particles, to which the colloidal condition is due. In 

 the case of most emulsoids and of a few of the suspensoids, this 

 change converts the mass into a stiff gel; but in that of many 

 of the metallic suspensoids, the dispersed particles are gathered 

 together into larger aggregates, which settle out of the liquid in 

 the form of a gelatinous precipitate. In the latter case, the effect 

 is usually spoken of as " precipitation " by electrolytes; while in 

 the former, it is called " coagulation," or " gelation." 



The effectiveness of the various electrolytes in bringing about 

 this change is proportional to their valency; bivalent ions are from 

 70 to 80 times, and trivalent ions about 600 times as effective as 

 monovalent ions. 



Further, all sols in which the dispersed particles carry a 

 charge of the opposite sign likewise precipitate both suspensoids 

 and emulsoids. 



A demonstration of the presence of an electric charge on the 

 particles of a sol and a determination of its sign can be made by 



