Wander to Kathode. 

 + 

 Metallic hydro-oxides generally. 

 Methyl violet. 

 Methylene blue. 

 Magdala red. 

 Titanic acid. 



COLLOit) CHEMlSl'RY. 207 



electrolysis. Spring,' Lottermoaer,- and others give the following lists as 

 to the behaviour of particles in aqueous sols :-- 



WandtT to Anode. 



Colloidal metals. 



Metallic sulphides. 



Silver halides. 



Sulphur. 



Selenium. 



Aniline blue. 



Indigo. 



Eosin. 



Fuchsin, 



Mastic. 



Many organic colloids, and some mineral ones {e.g., silicic acid), 

 wander to the anode in alkaline and the cathode in acid solution, while 

 certain neutral substances in water, and many others in suitable mixtures 

 of water and alcohol, are electrically indifferent. Definite charge in 

 relation to the medium is not therefore essential to the colloid state, 

 though it appears to be so to electrolytic flocculation. 



From what has just been said, it may be concluded that colloidal 

 solutions must possess a sort of pseudo-electrolytic conductivity ; but if 

 this is the case, the amount is so slight that it is difficult to decide 

 whether it depends on the colloid particles or on the residual traces of 

 electrolyte, from which it is impossible to free the sols, as it but slightly 

 exceeds that of the purest attainable water. Undoubtedly the particles 

 do carry charges ; but the charges are small, and the motion slow, and 

 the number of moving particles very small compared with the ions in an 

 ordinary electrolyte. It is also uncertain whether the j^articles are 

 actually discharged at the electrodes, though occasionally particles appear 

 to be repelled from both poles, and to collect in the middle of the 

 liquid, which may be due to those which have reached the electrode 

 and received a contrary charge wandering backwards, and encountering 

 and attracting originally charged particles proceeding in the opposite 

 direction. 



All inorganic sols are flocculated and precipitated by the addition of 

 electrolytes, though they vary in sensitiveness. Oi-ganic sols are much 

 less sensitive in this respect, but are frequently flocculated by the 

 addition of organic solvents such as ether or alcohol, to which inorganic 

 sols are usually indiflerent. The precipitate is usually termed the ' gel,' 

 but the term must not be confounded with 'jelly ' to which many of the 

 gels are but distantly related. Gels may be ' reversible,' and may readily 

 return to the sol state on restoring the original conditions, such as 

 removing the added electrolytes ; but frequently they are ' irreversible,' 

 that is, the sols cannot be restored without going through the oftea 

 complicated operations by which they were originally produced. 



While the phenomenon of flocculation is apparently similar in in- 

 organic and organic sols, it probably often difiers rather widely in its 

 causes, that of inorganic sols being closely connected with the electric 

 chai'ges of the particles ; while in organic sols, osmotic effects which will 

 be best discussed in connection with the properties of jellies, have 



' Bull. Acad. Roy. Be.lg. (3), 1898, 35, 780, 

 ' Anorganitche Kolloide, p. 76. 



