10 PROCEEDINGS OF THE AMERICAN ACADEMY. 



prepared in yellow light, and all excepting the third were allowed to 

 stand nearly three weeks in total darkness with occasional shaking before 

 being filtered. Upon the expiration of this time thirty cubic centimeters 

 of each of the filtrates were heated and treated with an excess of a solu- 

 tion of hydric sulphide, the zincic solution being acidified with hydro- 

 chloric acid. The precipitates were allowed to settle upon the bottoms of 

 test tubes, where a very fair guess as to their relative quantities could be 

 obtained. The three normal potassic chloride solutions and the hydro- 

 chloric acid appeared to yield equal amounts of mercuric sulphide, the 

 zincic chloride very slightly less, while the deciuormal solution of 

 potassic chloride deposited no precipitate, and showed only the faintest 

 possible coloration. These experiments showed that: — 



(a) The reaction HgoCl2 = HgCL + Hg is an undoubted fact. 



(b) Light has no considerable effect on this reaction. 



(c) Dissolved air is also not the essential factor. 



(d) Different chlorides produce effects not very different. 



(e) The concentration of the chlorine ions in solution is a most im2:)or- 

 tant factor. 



(_/) About a decigram of mercuric chloride is formed in a litre of a 

 normal solution of potassic chloride at 20°. 



The equilibrium is an interesting one ; it will receive further attention 

 in this laboratory. 



That this mercuric salt actually increases the potential of the electrode 

 containing it was easily proved by adding mercuric chloride to a deci- 

 uormal calomel electrode, and comj^aring this electrode with a similar 

 one in which the alkaline chloride had the same concentration. Invari- 

 ably the mercuric salt raised the potential of the electrode in question. 

 After long standing this difference gradually lessened, as the mercuric 

 chloride was reduced ; but upon shaking the difference at once appeared 

 again. Undoubtedly in time all the excess of mercuric chloride would 

 be reduced, until the constant amount which seems to be in equilibrium 

 with mercnrous chloride and mercury under the given conditions was 

 reached, when the difference would disappear at constant temperature. 

 The chief conditions of experiment being now determined, the next step 

 was to make an accurate series of determinations of the temperature coeflJi- 

 cient of the potential of the cells Hg - HgCl - MCI,, - HgCl - Hg. 



Since low temperatures had been found less likely to cause the dis- 

 turbing formation of mercuric salt, the two temperatures 0° and 30° were 

 selected as the most suitable for the present purpose. Since the degree 

 of dilution had been found to produce such an important difference in 



