301 



Mr. H. A. McTaggart on the 



valency of one of its ions. That ion which is o£ the opposite 

 electrical sign to the colloidal particle is the one which 

 causes the coagulation, i. e. it is the ion which annuls the 

 charge on the colloid. In endosmose experiments with 

 porous diaphragms, Perrin {Journ. de Chim. 8f Phys, ii. 1904, 

 iii. 1905) finds a similar rule to hold regarding the activity 

 or an ion in discharging the solid-liquid surface. A trivalent 

 ion is much more active in this respect than a monovalent 

 ion. A positive ion reduces the charge on a negatively 

 charged surface and conversely. In general, at a liquid- solid 

 surface, the solid is charged negatively in a basic solution, 

 and positively in an acid solution. It will be seen that these 

 rules apply, in a general way, to liquid-gas surfaces as well. 



The two acids HC1 and HN0 3 , having the monovalent 

 positively charged hydrogen ion, were tried in very dilute 

 solutions. With increasing concentration the negative 

 charge on a bubble of air was reduced until it was practically 

 neutralized. Very strong solutions could not be used 

 because of the disturbing effect of gases given off at the 

 electrodes. In the highest concentration that was used, 

 however, no reversal of the sign was found. HG1 was used 

 in concentrations from 10 -7 equivalents per c.c, to 10~ 5 

 equivalents per c.c. as estimated against standard baryta- 

 water. HN0 3 over a similar range of concentrations gave 

 practically the same results as HC1. 



To obtain, if possible, some examples of a reversal of the 

 sign of the charge, a number of trivalent positive ions were 

 chosen. Readings for three nitrates are given in the tables 

 below. In the first column are the concentrations in equi- 

 valents per c.c, in the second the time taken by a bubble of 

 air in moving over a fixed distance, and in the third the sign 

 of its charge. When the velocities are too slow to be timed 

 conveniently the words " slow/' - ; faster," are used to give a 

 general idea of the motion. 



La(]S T 3 ) 3 6 H 2 0. 



Time. 



Charge. 





Seconds. 





KT 9 .7-5 



40 



— 



10^ 8 . 7'5 



64 



— 



10~ 7 .3-7 



240 



— 



10 " 7 . 7-5 



— 







10~ 6 . 3-7 



slow 



■f 



