ASPECTS OF ADSORPTION PHENOMENA 197 



Equal weights of the following samples of filter-paper were 

 placed in 50 c.c. distilled water with 10 c.c. Q dye as usual. The 

 amounts taken up by each are given : 



1. Dreverhof's ordinary, No. 333 ... ... 95 % 



2. washed in distilled water 59 % 



3. Dreverhof's extracted paper ... ... ... 46 % 



4- washed 46 % 



5. Schleicher and Schull's extracted paper ... 46 % 



I may mention here that Dreverhof's extracted paper turns blue 

 when a drop of dilute congo-red is placed on it, this does not happen 

 with Schleicher and Scholl's, nor with Swedish paper. As the above 

 experiment shows this slight acidity does not seem to affect its 

 adsorbing power, and as will be seen later H+ ions have comparatively 

 little favouring effect. 



The result of this experiment seems to suggest the possibility, 

 that if one could obtain a complete absence of electrolytes no adsorption 

 would take place. 



As regards trivalent kations I have only tested aluminium 

 sulphate, but as even in a concentration of precipitation took 



place the result was valueless. 



It is scarcely necessary, perhaps, to remark that if the dye is pre- 

 cipitated by the electrolyte no adsorption by the paper takes place, the 

 large particles merely float about in the liquid. What is needed is 

 the local concentration of electrolyte on the surfaces of the adsorbing 



solids. 1 



Although the greater favouring power of divalent as compared with 



monovalent kations is easy to demonstrate the order in each of these 

 classes is difficult to decide since the action of each member is so 

 nearly the same as that of the others. From a number of experiments 

 the following seems to be the order :. 



H < Li < K < Na < NH 4 < Mg < Ca 



i. Notwithstanding the fact that actual precipitation must not take place in these experiments the 

 addition of electrolytes to congo-red, for example, causes an increase in the size of the colloidal particles 

 even vvh-n no actual precipitation occurs ; so that the solution is on its way to precipitation even when 

 this does not actually occur. The specimen of congo-red used in all my experiments showed the 

 Tyndall phenomena very faintly in solution in distilled water ; but when NaCl was added the beam of 

 light became much more diitinct and the light reflected from it at right angles was polarized. When 

 CaQ 2 was added in sufficient amount to slowly precipitate the dye (about ) the polarization ceased 

 as the particles became larger than the mean move-length of light. 



