392 THE COLLOIDAL STATE 



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. 



(2) The Precipitation of Colloids by Other Colloids of Oppo- 

 site Electric Sign. — This phenomenon was first observed by 

 Linder and Picton, who found that certain solutions of organic 

 dyes, on mixing, produced precipitates. Further investiga- 

 tions have shown conclusively that only oppositely charged 

 colloids could mutually precipitate ; thus, arsenic sulphide, 

 which is negatively charged, is not precipitated by any 

 other negatively charged colloid, but is precipitated by 

 ferric hydroxide, which is positive. The resulting gel 

 is described as an adsorption compound (see below under 

 Adsorption). 



This mutual precipitation of colloids has many very im- 

 portant practical applications ; for example, the use of ferric 

 salts in the purification of sewage water is probably due to 

 the precipitation of negatively charged colloidal particles of 

 sewage by the ferric hydroxide hydrosol and similarly with 

 alum. 



Also it has been suggested that the process of dyeing 

 is really a mutual gel formation between the colloidal dye and 

 the colloidal fibre ; similarly the interaction between toxin 

 and antitoxin, and the phenomenon of bacterial agglutination, 

 etc., may be regarded as examples of the mutual precipitation 

 of two colloids. 



This same phenomenon can also be conveniently employed 

 for determining the electric sign of a colloid. Thus, if a piece 

 of filter paper is wetted, it assumes a negative charge and con- 

 sequently if it is dipped into a positive dye sol the dye will 

 be discharged on coming in contact with the paper, and water 

 alone will be drawn up by capillary forces. If, on the other 

 hand, the dye is a negatively charged one it will travel up the 

 paper together with the water. This may be well shown by 



