ASPECTS OF ADSORPTION PHENOMENA 195 



When adsorption has taken place under the action of electrolytes 

 a very considerable degree of fixation occurs, so that the dye is not 

 given up again to water, or only to a very slight degree. 



Experiment: Two similar pieces of paper dyed to same depth of 

 colour, one in solution of Nad 0.02 per cent., the other in distilled 

 water. Of course the latter solution was more concentrated as regards 

 the dye. These pieces were rinsed in water, pressed between blotting- 

 paper, and then placed in equal volumes of distilled water along with 

 another piece of paper each. After 24 hours the one which had been 

 dyed in distilled water had given up a considerable amount of pigment 

 to the water, and the two pieces of paper were very nearly equal in 

 depth of tint. The paper dyed in the" presence of Naci gave up no 

 colour to be detected by the eye, although the second piece of paper 

 was faintly pink, showing a slight extraction from the dyed piece. 



IV. THE ACTION OF ELECTROLYTES 



When we consider that a solution containing only 0^0005 per 

 cent, of Naci has a distinct effect in augmenting adsorption of congo- 

 red by paper, viz., 23 per cent, taken up as against 16 per cent, from 

 distilled water, it is plain that the phenomenon is not of the nature of 

 a salting-out,' comparable to the precipitation say of egg-albumen by 

 ammonium sulphate. The precipitation of such solutions as those of 

 the colloidal metals and hydroxides is rather suggested. Now, there 

 is considerable evidence that a large number of the anilin dyes exist in 

 watery solutions in a colloidal form. Congo-red, having a molecular 

 weight of nearly 700, would be expected to be one of these. Its 

 solutions, in fact, do not diffuse through Schleicher and Schilll's 

 parchment-paper thimbles, although they do so, very slowly, through 

 ordinary parchment-paper. According to Michaelis 1 under the ultra- 

 microscope they are heterogeneous, being resolvable into sub- 

 microscopic particles. In the electric field the dye migrates to the 

 anode, so that the particles are negatively charged. It is, however, 

 somewhat difficult to make oneself certain of this fact on account of 



i. Deutsche Med, Wochensckrift, 1904, No. 42. 



