12 REPORTS ON THE ST! 
sodium salt with thé most strongly adsorbed anion will be the 
most effective in causing precipitation. Negatively charged 
albumin is precipitated readily by sodium chloride, and not at 
all by sodium iodide, while positively charged albumin is precipi- 
tated by sodium jodide much more readily than by sodium 
chloride. 
If one over-runs the point of electrical neutrality, the suspen- 
sion may become stable again because it is stabilised by preferen- 
tial adsorption of an ion “having the opposite sign. If one adds 
a little less than the equivalent amount of a dilute potassium 
bromide solution to a dilute silver nitrate solution, one gets a 
positively-charged colloidal solution of silver bromide because 
the silver bromide is stabilised by adsorbed silver ions. If one 
adds slightly more than the equivalent amount of the potassium 
bromide solution, there is obtained a negatively-charged colloidal 
solution of silver bromide, stabilised by adsorbed bromine ions. 
If more bromide solution is added, the silver bromide will coagu- 
late because there will come a point at which the slight adsorp- 
tion of the potassium ion counterbalances the adsorption of the 
bromine ion. In other words, silver bromide precipitates, when 
there is a distinct excess of silver nitrate, stays in suspension as a 
positively charged colloid when there is only a slight excess of 
silver nitrate, precipitates, when the suspension becomes electri- 
cally neutral, stays in suspension as a negatively charged colloid 
when there is a slight excess of potassium bromide, and precip1- 
tates when there is a distinct excess of potassium bromide. At 
the two ends of this series there is also the possibility of true 
solution of silver bromide in silver nitrate or potassium bromide. 
An interesting case of the precipitation of a colloid by an 
electrolyte is to be found in the action of sea-water on muddy 
river water. Skey** pointed out that suspended mud is precipi- 
tated by electrolytes, and Waldie’® has discussed the clearing of 
the water of the Hooghly. Whén a river flows into the ocean, the 
current becomes less, and sonie of the suspended mud is dropped 
on this account; but that is relatively unimportant in many cases. 
Schloesing*® called attention to the fact that the formation of 
deltas is due primarily to the coagulation of the suspension by 
the electrolytes in the salt water. Another interesting case of 
the neutralisation of an electrical charge is the precipitation of 
positively charged colloids by 8 rays.” 
Since the important thing in the neutralisation of an adsorbed 
ion is the adsorption of an ion of the opposite charge, we may 
get neutralisation when we have a colloid with the opposite 
charge. In other words, we may neutralise an adsorbed ion 
with another adsorbed ion instead of by a free ion. The usual 
statement is that sols having the same charge do not affect each 
other perceptibly, while sols having opposite charges precipitate 
™ Chem. News, 1'7, 160 (1868). 
75 Chem. News, 30, 37 (1874). 
76 Jour. Chem. Soc., 24, 750 (1871). 
7 Hardy, Jour. Physiology, 29, 29 (1903). Hober, Physikalische Chemie der 
Zelle und Zewebe, 332 (1911); Burton, Lhe Physical Properties of Colloidal 
Solutions, 172 (1916). 
