206 BIO-CHEMICAL JOURNAL 



The conclusion is, I think, justified that paper takes up very little 

 electro-negative dye because it is itself negatively charged, and that 

 when the dye is discharged by the addition of an electro positive colloid 

 or a kation, there is no longer the same mutual repulsion between the 

 dye and paper. 



It would be of interest to test the behaviour of paper in turpentine, 

 in which it would be electro-positive, if one could obtain an electro- 

 negative colloidal dye in solution in turpentine. 



This interpretation is confirmed by the results of experiments on 

 silk :- 



Experiment : From a watery solution of congo-red a piece of 

 paper took up 26 per cent., a piece of silk (well washed), of the same 



weight took up only 5 per cent. From a ~ Mgso 4 solution, paper took 

 up 91 per cent, and silk 98 per cent. From toluidin-blue in water, 

 paper took up 85 per cent. Silk rapidly took up the whole, so that I 

 added another double amount of dye solution, the colorimeter reading 

 thus showed that 67 per cent, of the whole, that is 67 x 3 = 201 as 

 compared with 85 in the case of paper was taken up. The action of 

 neutral-salts on the adsorption of positive dyes by silk was of the 

 opposite sign to their effect on paper. For example : 



Paper : From water ... ... 85 % adsorbed 



-^ MgS0 4 5 I % 



Silk : From water (concentration of dye three times 

 that in case of paper) 67 % adsorbed 



The results may be explained by the consideration that silk, owing 

 to its lower dielectric constant than that of paper, would no doubt have 

 a higher negative charge, which would make it less accessible to 

 negatively-charged dyes, but more so to positively-charged dyes. I 

 am unable to state why the action of electrolytes on the positive dye 

 is opposite in the two cases of silk and paper, unless that for some 

 reason silk is relatively more sensitive to the adjuvant action of the 

 anion. If this were so, however, one would expect a more marked 



