CH. n] THE COLLOIDAL STATE 11 



Hence two terms have been employed for the above-mentioned 

 types of colloidal solutions: those of gold, silver, etc., are termed suspen- 

 soids (suspensoid sols) : those of starch, proteins, etc., emulsoids (emul- 

 soid sols). 



The essential feature of both forms is that they are systems consisting 

 of two phases, or conditions of matter, known respectively as the " dis- 

 persed " phase and the " continuous " phase. 



A suspensoid may be denned as having a dispersed phase composed 

 of ultramicroscopic particles or aggregates of molecules suspended in a 

 continuous phase composed of a liquid. 



An emulsoid may be defined as having a dispersed phase composed 

 of ultramicroscopic drops of a highly concentrated solution of the sub- 

 stance suspended in a continuous phase composed of a dilute solution of 

 the same substance. 



As a rule, therefore, the difference between a suspensoid and an 

 emulsoid is that, whereas in the former the liquid is restricted to the 

 continuous phase, and the solid to the dispersed phase, in an emulsoid 

 both phases are liquid, though containing different proportions of the 

 dissolved substance. 



The terms suspensoid and emulsoid are used on account of the re- 

 semblance of these states of matter respectively to suspensions and 

 emulsions. If microscopic particles of a solid are shaken up in water, 

 what is known as a suspension is obtained ; in time, however, the solid 

 particles, if heavy enough, will settle and separate from the water, and 

 the whole process can be repeated. Thus a suspension differs from a 

 suspensoid solution in that the latter is stable, though, if precipitated, 

 the reaction is usually not reversible. 



If two liquids which are insoluble in each other, such as oil and 

 water, are shaken up together, finely divided drops of oil in water are 

 obtained. This is known as an emulsion. In time, however, the oil 

 separates from the water, because the tension on the films of water 

 separating the oil drops, when in contact, is too great, and they break, 

 with the result that the oil drops coalesce. But if, instead of water, a 

 solution of soap, saponins, or certain other substances is used, the surface 

 tension of the water is so lowered that the films of soap solution separating 

 the oil drops are permanent, and a system is obtained consisting of minute 

 drops of oil separated by soap solution. This system resembles an organic 

 colloidal solution, as, for instance, that of protein in which we suppose 

 a concentrated solution of protein exists in drops separated by a dilute 

 solution of protein. Milk and latex constitute natural emulsions. 



