COLLOID THEORY. 15 



the particles which lead to flocculations among- the sticky particles, 

 just as many precipitates are flocculated by mechanical agitation or 

 boiling. 



If by the influence of light more viscous film is formed on the light 

 side of the suspended particles than on the shaded side the particles 

 will migrate toward the light that is, be positively photodromic. 

 Under the contrary supposition they would go to the shaded side, or 

 be negatively photodromic. If the viscous films contained to any 

 extent occluded gases as well as other substances, foam cells would 

 form, accounting not only for the indefinite suspension of the particle, 

 but for the migratory effects under the influence of light.- This phase 

 of the subject is important and requires further investigation, as the 

 photodromic effects are probably active on a large scale in nature. 



Linder and Picton 05 have shown that when a coagulated colloidal 

 precipitate is formed in the presence of an electrolyte a portion of the 

 basic ions of the electrolyte is absorbed by or adsorbed on the pec- 

 toid matter in the form of the hydroxid, while the acid ions remain 

 free in the solution. Many other investigators have checked this 

 astonishing and most interesting fact. Whitney and Ober & repeated 

 the work in the following manner: A colloidal solution of arsenious 

 sulphid was coagulated b}^ addition of dilute neutral solution of barium 

 chlorid of known strength. After filtration and thorough washing a 

 portion of the barium as hydroxid was found to have been held by the 

 precipitate, while all of the chlorin was accounted for in the clear 

 filtrate, which was now acid with hydrochloric acid formed by the 

 hydrolysis. Furthermore, if the colloid precipitate was subsequently 

 digested with a dilute solution of ammonic chlorid all the barium 

 hydroxid was again released and passed into solution. This is an 

 important point which will be considered in its bearing upon rock 

 powders later on. Van Bemmelen c has prepared artificial hydrogels 

 of silicic acid, aluminum silicate, calcium silicates, etc., and has found 

 that generally when these substances form, absorption of the basic ions 

 takes place, with an accompanying induced acidity in the clear filtrates. 



It appears that in certain cases the base will be absorbed from solu- 

 tions of electrolytes by pectoid substances, the acid ion remaining 

 free in the solution. In other cases the acid will be taken and the 

 base left, and again the whole salt will be occluded. This absorption 

 po\ver is in no sense of the word due to chemical combination, but 

 seems to depend upon the fine submicroscopic porosity of the pectoid 

 characterized by Van Bemmelen as " Struktur-Membran mit offenem 



J. Chem. Soc. (Lond.), 1893, 61 : 114, 148. 

 &J. Amer. Chem. Soc., 1901, 23 : 842. 

 . anorg. Chem., 1900, 23 : 358. 



