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Mr. J. A. Craw. On the Filtration of [Dec. 1, 



very slowly, if at all. These facts lend much probability to Oswald's view 

 that colloidal solutions may be regarded as very fine suspensions. As the 

 molecular weights of these substances are as yet quite indefinite, it would 

 seem more justifiable to speak of the colloidal " grain " than of the colloidal 

 "molecule." Accepting this view of the nature of colloidal solutions, the 

 action of the filter admits of an apparently satisfactory explanation on a 

 purely mechanical basis.* During the filtration, for example, of colloidal 

 ferric hydrate, Table II, the first fraction contained none of the colloid, and 

 probably consisted chiefly of the water of the filter. In the succeeding 

 fractions a little ferric hydrate passed through the larger pores, but these were 

 speedily blocked by the particles. Under the pressure of 100 atmospheres 

 the pores are probably compressed, and the effective passages are soon com- 

 pletely blocked by ferric hydrate. On suddenly diminishing the pressure the 

 gelatine begins to return to its original condition, and the effective passages 

 increase in number and diameter. The condensed ferric hydrate, no longer 

 completely blocking the widened passages, is swept out of the gelatine by the 

 oncoming fluid, and so gives a filtrate with an abnormally high concen- 

 tration. 



On the other hand, by gradually diminishing the pressure the ferric 

 hydrate has time to fill the new and the gradually-widening pores, so that 

 the filtrate contains only a trace of colloid. If the ferric hydrate be not- 

 expressed on recompression, it is conceivable that the compound filter of 

 ferric hydrate and gelatine would be less affected as regards permeability by 

 variation of pressure than the original gelatine. Much as this view has to 

 recommend it, it does not seem to be the whole explanation, as it does not 

 satisfactorily account for the phenomena observed with sodium chloride. It 

 is highly improbable that sodium chloride is retained by a mechanical 

 blocking of the passages, and that it is swept in high concentration into 

 the filtrate, on decompression, because the condensed sodium chloride is no 

 longer of the same dimensions as the widened passages. Eecourse must, 

 therefore, be had to some other view, which may considerably modify the 

 explanation given for colloidal substances. 



Adsorption Hypotheses. — Porous substances, powders, and fine suspensions 

 of solids in aqueous media have the power of removing salts and other 

 substances from solution. The action is attributed to forces of the same 

 nature as those which give rise to adhesion and the wetting of a solid by 

 a liquid, e.g., of clean glass by water. The phenomenon usually called 

 adsorption is common to all substances with highly developed surfaces, and 



* Cf. also M. Traube (1866, 1867). Traube regards precipitation membranes as atomic 

 and molecular sieves. 



