PHYSICAL FORCES INVOLVED IN BIOLOGICAL ACTIVITIES 179 



illustrated by reference to casein in milk, or gelatin in aqueous solution, 

 which is easily grasped to differ from a solution of salt, a crystalloid, in 

 water. In colloidal solutions the particles are referred to as the disperse 

 phase, the medium in which they are found, the dispersion means and 

 the solution as a whole, a dispersoid. In the event that gold be reduced 

 so fine that its suspension gives rise to a colloidal solution, gold would be 

 the disperse phase, water the dispersion means and the solution or 

 suspension as a whole, the dispersoid. The gold would also represent 

 one phase and the water another phase, resulting in a diphasic hetero- 

 geneous system. Where the gold particle and the water meet or at the 

 point the disperse phase and dispersion means come together or are in 

 contact is the so-called interface so important in surface energy. Some- 

 times the disperse phase is called the internal phase and the dispersion 

 means the external or continuous phase. 



Dispersoids exist as suspension-colloids or suspensoids and emulsion- 

 colloids or emulsoids. The former designate the disperse phase to be a 

 solid and the dispersion means a liquid (lyophobic colloids)', the latter 

 designate the disperse phase to be a liquid and the dispersion means also 

 a liquid (lyophilic colloids). As an example of the former, colloidal gold 

 as the disperse phase and water as the dispersion means is satisfactorily 

 typical; as an example of the latter, gelatin as the disperse phase and 

 water as the dispersion means qualifies, although the gelatin is very 

 close to a solid at times but probably still in a hydrated condition. 



This attempt to divide the colloidal condition or state into two 

 classes is quite general. In the above paragraph Von Weimarn* and 

 Ostwald have made the division into suspensoids and emulsoids, 

 Perrinf into lyophobic and lyophilic. NoyesJ contributes another 

 division: "As types of these I would draw your attention to these 

 aqueous solutions of gelatin and of colloidal arsenious sulphide. The 

 former class possesses a much greater viscosity than that of water; the 

 latter does not appreciably differ from it in this respect. The former 

 gelatinizes upon cooling or upon evaporation, and passes again into 

 solution upon heating or addition of the solvent; the latter does not 

 gelatinize upon cooling, and if gelatinized by other means it does not 

 redissolve upon heating. The former is not coagulated by the addition 

 of salts (unless in excessive amount), the latter immediately gives an 



* Von Weimarn, Grundzuge der Dispersoid Chemie (Steinkopff, Dresden), 1911. 



t Perrin, J., J. Chim. Phys., 3, 5O, 1905. 



J Noyes, A. A., Jour. Amer. Chem. Soc. 27, 2. p. 85, 1905. 



