4 A. VV. GREELEY. 



tides), which differ from the dissolved substance in a crystalloidal 

 solution in that they are relatively very large aggregates of 

 molecules of the colloidal matter. These particles do not affect 

 the osmotic pressure of the fluid matrix ; while in a crystalloidal 

 solution the solute exists in a molecular or ionic condition, and 

 thus gives to the solution a definite osmotic pressure. 



2. The physical state of the entire solution depends on the 

 condition of these colloidal particles. When they are finely 

 divided and separated from each other in the solvent, the colloid 

 appears as a liquid or exists in the " sol ' phase. If, however, 

 the colloidal particles become fused, and thus lose their condition 

 of fine suspension, the colloid becomes relatively solid, or passes 

 into the " gel " phase. 



3. The physical state of the colloidal particles and hence of 

 the entire solution varies directly with certain external conditions. 

 Thus the passage from the " sol ' into the " gel " phase is 

 accomplished in the following ways : (A) by variations in the 

 temperature, (BJ by chemical .changes, (C) by the action of the 

 electric current. 



A. The physical state of the organic colloids varies directly 

 with the temperature. If the "sol" phase is constant at the 

 normal temperature, 20 C., coagulation gradually takes place 

 as the temperature is lowered, until at o C. almost complete 

 gelation has occurred. As the temperature is raised above the 

 normal the fluidity of the solution is increased (by the subdivision 

 of the colloidal particles and absorption of water), until a critical 

 point is reached at which coagulation suddenly occurs, and the 

 colloid goes into a condition of heat rigor. 



B. A colloid may be coagulated by adding to it a solution of 

 any electrolyte which bears an electrical charge opposite in sign 

 to that carried by the colloidal particles.* Thus Hardy found that 

 a positively charged colloidal solution was coagulated by any 

 electrolyte with a powerful anion or negatively charged ion, and 

 the rapidity of the coagulation varied directly with the valence of 

 the anion. A negatively charged colloid was coagulated by 

 electrolytes of an opposite electrical character, or by powerful 

 cathions. Similarly an electrolyte carrying a charge of the same 

 sign as that of the colloidal particles causes them to subdivide 



