RICHARDS. 



CALOMEL ELECTRODE, 



SERIES V. 



Bromides between 0° and 30°. 



Hg - HgBr - (MBr„) - HgBr - Hg. 



It was at once evident that the cause of the difficulty was the occur- 

 rence of the following reactions : — 



2HgCl* = HgCl, + Hg, 

 2 HgBr = IlgBi-a + Hg, 



which seem to be effected by a " catalytic " action of the haloid ions in 

 the solution. 



This reaction has already been observed by Miahle,t but a confirma- 

 tion seemed desirable. Numerous solutions decanted from the present 

 experiments were filtered, and found by testing with stannous chloride or 

 hydrogen sulphide to contain mercuric salts, — the normal bromides very 

 large amounts, the normal chlorides and decinormal bromides much less, 

 and the decinormal chlorides very little. In order to subject the method 

 to a rigid test, a quantity of mercurous chloride was prepared with very 

 great precautions, being made and kept wholly in non-actinic light 

 or darkness. This material after very thorough washing was divided 

 between a number of bottles, to each of which was added a carefully 

 made standard solution of some chloride. Three bottles of normal po- 

 tassic chloride were made in this way, one being carefully boiled, wholly 

 filled, while hot, hermetically stoppered, and kept in perfect darkness, 

 another being half filled and left in contact with air, but also in dark- 

 ness, and the third, otherwise like the second,- being exposed to bright 

 sunlight. Bottles similarly filled with ^4) decinormal potassic chloride, 

 (5) normal zincic chloride, and (6) normal hydrochloric acid, were also 



* The adoption of HgCl instead of HgsCla as the formula of mercurous chloride 

 is purely for convenience. The true formula of this substance in the solid state 

 seems to me doubtful. 



t J. Pharm., XXVI. 108; Ann. Chim. et Phys., (3.), V. 177. 



