SUSPENSOIDS 289 



and consequently wander towar.ds 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 electrolyte ; 

 the precipitating power of such ions is a function of the valency 

 and therefore in the order shown by the following series : 

 -NaCl, BaCl 2 , A1C1 8 . 



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 

 of sea salts upon the positively charged suspended clay 

 particles is an illustration on a large scale of an analogous 

 phenomenon. 



Positively charged colloids, such as ferric hydroxide, on 

 the other hand, are precipitated by the anion of an electrolyte, 

 the precipitating power again increasing with the valency as 

 indicated by the series sodium chloride, sulphate, citrate. 



19 



