ASPECTS OF ADSORPTION PHENOMENA 203 



In order to save space, I have put together in a table at the end 

 of this paper a number of facts relating to various dyes in common 

 use which have a bearing on the question under discussion. 



The only other point 1 will refer to as regards the negative dyes 

 is that eosin, although its colloidal properties are doubtful (see table), 

 shows a much greater sensitiveness to the anion than the dyes hitherto 

 mentioned, alkali in this case causes the taking up of less colour by 

 paper than is taken up from water. We found in the previous cases 

 that the action of the o ^ { ion was overpowered by the opposite action 

 of the kation ( N *). 



We turn now to the electro-positive dyes like toluidin-blue. It 

 would be expected that in this instance the anions would have a 

 favouring action, the kations the reverse. This is, indeed, the case, 

 but like the negative dyes the effect of the kation is predominant, so 

 that the result of adding any electrolytes, except alkalies, is to lessen 

 adsorption. In the absence of electrolytes, as a rule, more dye is 

 taken up by the paper than from the solution of an electro-negative 

 dye. This would appear to be at variance with the general use of 

 congo-red to dye cotton, but it it is to be remembered that in 

 practice there is always sufficient electrolyte present to reverse the 

 relative behaviour of negative and positive dyes as shown when 

 dissolved in pure distilled water. 



For details as to various positive dyes see the table. 



Methylene blue precipitates the electro-negative colloidal platinum. 



I have not been able to detect any marked protective action of 

 stable colloids in the case of electro-positive dyes. 



In the circumstance that both the + and dyes are more 

 sensitive to kations than to anions they behave like albumin solutions 

 to salts of the heavy metals as shown by Pauli. 1 



On reference to the table at the end of this paper it will be 

 noticed that in all the cases investigated where the dye is in the form 

 of a salt with an inorganic base or acid, as the case may be, the sign of 

 the charge of the colloid is determined by the crganic constituent of 



j. Hofmeister's Beitrage, VI, pp. 233 10249. 



