COLLOIDS. 47 



which tries to diminish the total contact surface between the two media, 

 which can happen by the small particles uniting to form larger ones, 

 when flocking is brought about. The electrical charge of the particles 

 acts against the surface tension so that equally charged particles repel 

 each other. If the electrical charge is discharged, as takes place in the 

 isoelectric point, then the surface tension reaches its highest value and 

 the precipitation may occur. 



The correctness of HARDY'S claim that precipitation occurs just in 

 the isoelectric fluid is disputed on special grounds by BILLITZER. He 

 believes that the ions have a much greater charge than the colloid par- 

 ticles. An ion collects the oppositely charged colloid particles around 

 itself and during these neutralization processes it may occur that the 

 entire complex may become so great that it becomes evident and on 

 account of the gravity it precipitates out. 



In general it can be stated that the stability of a colloid is greater the smaller, 

 cet.par., the particles are; as the probability that the number of particles sufficient 

 for the precipitation is then less. With equal size of particles the stability of a 

 colloid is dependent upon the size of the charge which the particles carry. Too 

 weak and very strongly charged colloids are relatively more stable; the first 

 because of the large number which must collect around an ion when flocking takes 

 place and the second because the number of particles required for the neutrali- 

 zation is perhaps too small, so that the necessary size of the complex for precipita- 

 tion is not attained. 1 



The findings of LINDER and PiCTON 2 that when colloidal As2&3 is 

 precipitated with BaCl 2 the solution becomes acid, and a small quantity 

 of barium remains in the precipitate, corresponds to BILLITZER' s theory. 

 This quantity of barium cannot be removed by water, but can be replaced 

 by the corresponding cation by washing with a solution of another salt. 

 According to BILLITZER in the mutual precipitation of colloids a quan- 

 tity relation exists which is dependent upon the electrical charges 3 

 (see also page 44). 



The fact that the precipitation of colloids is a manifestation of 

 processes which occur in a homogeneous medium, makes the understand- 

 ing of these especially difficult. If, as is generally accepted, we consider 

 the colloid solution as a homogeneous fluid of suspended solid or fluid 

 particles, then in the " solution" there occur at least two special con- 

 stituents, separated from each other the colloid particles and the sol- 

 vent. This is expressed as follows: the system contains two phases. 

 The solvent is often more correctly called the dispersion means and the 

 colloid particles called the disperse phase. If to such a system a new 



1 Zeitschr. f. physik. Chem., 45, 327, 1904; 51, 129, 1905. 



2 Journ. Chem. Soc., 67, 63, 1895. 



3 Zeitschr. f. physik. Chem., 51, 141, 1905. 



