PROPERTIES 391 



into it two platinum electrodes with a potential difference of 

 about 200 volts, the sol wanders to one or other of the elec- 

 trodes, showing that it bears an electrostatic charge of opposite 

 sign to that of the electrode in question. This phenomenon 

 is known as Kataphoresis. 



The majority of suspensoid sols bear a negative charge 

 and consequently wander towards the anode ; on the other 

 hand, the metallic hydroxides, silicic acid and basic dyes, etc., 

 wander to the cathode. While these statements are true for 

 aqueous sols the conditions are exactly reversed when turpen- 

 tine is the medium. This reversal of charge with the solvent 

 is governed by the rule that " non-conductors in contact with 

 a liquid assume a + or — charge according as their dielectric 

 constant is > or < that of the liquid." 



Since water has a very high dielectric constant it is natural 

 that most other substances should assume a negative charge 

 in relation to it. 



The fact that suspensoid sols bear a recognizable electric 

 charge renders them sensitive to electric influences, and they 

 are consequently readily discharged by colloids of opposite 

 sign or by electrolytes. This electrical discharge brings about 

 a coalescing of the colloidal particles, with the formation of 

 larger aggregates and consequent precipitation, resulting in 

 the destruction of the colloidal solution. Such a change is 

 irreversible, for the precipitate once formed cannot be re- 

 dissolved. 



(i) Precipitation by Electrolytes. — -The precipitation is in 

 this case, according to Hardy, effected by the ion of opposite 

 sign ; thus, for example, a negatively charged sol such as 

 arsenic sulphide is precipitated by the metallic ion of an elec- 

 trolyte ; the precipitating power of such ions increases with 

 the valency. 



That the metal really enters into close relationship with 

 the arsenic sulphide is shown by the fact that the latter when 

 precipitated persistently retains barium hydroxide whilst the 

 solution becomes acid due to liberation of hydrochloric 

 acid. 



The formation of a river delta by the precipitating action 



