ASPECTS OF ADSORPTION PHENOMENA 205 



sulphide for methyl-violet. We regard this fact as evidence 

 that the granular aggregates in these hydrogels still retain 

 some charge. In other words, the difference of potential existing 

 between the molecular aggregates and the field in the hydrosol state is 

 not destroyed by coagulants, but only reduced to a point at which the 

 forces of cohesion are just able to overpower the forces of repulsion 

 brought into play by such difference of potential.' 



It will readily be seen that if paper assumes a negative charge 

 when immersed in water the phenomena I have described above fall 

 into line with the behaviour of electrically charged colloids. We 

 notice that of a negative colloid like congo-red the less is taken up 

 by paper the more perfect the freedom of the water from electrolytes, 

 there is mutual repulsion until the dye is discharged by a kation. On 

 the other hand, the opposite holds as regards the positive dyes, there 

 is attraction of the negative paper for the positive dye. The kation 

 in this case probably acts by discharging the paper, being attracted 

 thereto by its opposite charge. 



Prof. Donnan, to whom I propounded the question, refers me 

 to the work of Coehn 1 , who found that when various non-conducting 

 bodies are immersed in fluids of different dielectric-constants they 

 assume a positive or negative charge according as their own dielectric- 

 constants are higher or lower than the fluid with which they are in 

 contact. For instance, glass (5-6) is negative in water (86) or alcohol 

 (26), whereas in turpentine (2-2) it is positive. Now, paper according 

 to Thwing 2 has a dielectric-constant of 2*82, and would therefore be 

 electrically negative to water. 



Moreover, Quincke 3 , in the course of his investigations on 

 electrical endosmosis found that, as a rule, all non-conducting bodies 

 in water took on a negative charge. Among the substances tested by 

 him were cotton-wool and silk ; particles of these in water exposed to 

 an electric field wandered to the anode and were therefore negatively 

 charged. 



i. PFied. Ann., 64, page 217, 1898. 



Zeitsch.f. Physik, C/temie.,XIV, page 292, 1894. 

 r. Ann. 113, page 583, 1861. 



