238 CHEMICAL DISCOVERY AND INVENTION 



by electrical dispersal of the metal itself below the surface of 

 water or other liquid. By making a small electric arc between 

 gold wires under water purple-red clouds and a coloured liquid 

 result from dispersal of the metal which comes from the cathode 

 only. It would not be surprising to find that the fine particles 

 of metal thus held in pseudo-solution are electrically charged, 

 but it is very significant of the character of colloids generally 

 to find that the particles of such substances as ferric hydroxide 

 and arsenious sulphide produced by the processes already men- 

 tioned are in this condition. The apparent attraction or repul- 

 sion of such substances in the colloidal state has been examined 

 by Messrs. Linder and Picton, who began their researches so far 

 back as 1892, and who must be regarded as among the chief 

 pioneers in this difficult field of enquiry. They describe the 

 electrical convection of arsenious sulphide as follows, using the 

 yellow liquid in which no particles are visible under the micro- 

 scope, while the liquid is filterable : 



" The resistance of such a solution is extremely high and the 

 current passing through it in one case amounted to only 0-000007 

 ampere. The conductivity is probably due to the presence of 

 small traces of arsenious oxide ; but, however that may be, the 

 passage of this small amount of current is accompanied by the 

 repulsion of the colloidal sulphide as a whole from the negative 

 electrode." (Trans. Chem. Soc., 1897, p. 569.) They also 

 observed that of different colloids examined some are positive 

 and some negative : thus arsenious sulphide is negative in a 

 solution faintly acid to litmus, while ferric hydroxide is electro- 

 positive under the same conditions. 



All these hydrosols or colloidal solutions are coagulated when 

 mixed with an electrolyte. It has already been mentioned that 

 the addition of a little hydrochloric acid to the arsenious sulphide 

 liquid causes immediate precipitation of the solid sulphide, but 

 an acid is by no means necessary, as in most cases any soluble 

 neutral salt will bring about the same effect. It is interesting 

 to notice that the coagulating effect is delayed or prevented 

 altogether in some cases by the protective effect of mixing the 

 hydrosol with one of the more stable colloids, such as a solution 

 of gelatine. Faraday discovered, for example, that his red gold 

 solution when mixed with jelly " is rendered much more per- 

 manent than before ; and then it may by a little warmth be had 

 in the fluid state, or by cooling as a tremulous jelly, or by desic- 



