of Electrolytic Decomposition. 43 



broken, apparently in consequence of the heat generated 

 at the + electrode volatilizing, or otherwise causing the 

 mercuric iodide to disappear from the porous pot. After 

 passing the current for some hours the gas was extinguished, 

 and when the apparatus had become cool the two branches of 

 the U-tube were cut asunder and broken open for examination. 

 In the + branch, both within and without the porous pot, 

 mercuric iodide and black feathery streaks of iodine were 

 found which gave the starch reaction, and the upper part of 

 the glass tube was coated with volatilized iodine. 



Iodine dissolves freely in fused mercuric iodide, producing 

 but little change in the colour of the latter ; but just as soli- 

 dification commences, more or less complete separation seems 

 to take place with the formation of these black patches rich 

 in iodine (and Hg 4 I 6 ?) and an evolution of iodine vapour, 

 which in the previous experiments also helped to lead to the 

 conclusion that electrolytic decomposition occurred only near 

 the melting-point of mercuric iodide. 



The contents of the negative branch and porous pot were found 

 to be of a slightly altered colour; but no free mercury was 

 discoverable. Repeated exhaustion of its pulverized contents 

 with absolute alcohol revealed the presence of mercuroso- mer- 

 curic iodide (Hg 4 I 6 or 2HgI 2 , Hg 2 I 2 ), which might perhaps 

 be inferred to be the first product of the electrolytic decom- 

 position of Hgl 2 , since it is formed when metallic mercury 

 and mercuric iodide are titurated together in the proper pro- 

 portions. Mercuroso-mercuric iodide is ordinarily regarded 

 as a distinct compound, and I suppose rightly so ; for it can 

 exist without decomposition at a temperature at which mer- 

 curous iodide undergoes decomposition with liberation of free 

 mercury ; but the stability of this body seems to be greatly 

 increased by the presence of a slight excess of mercuric iodide. 

 Mercurous iodide (Hg 2 I 2 ) dissolves readily in fused mercuric 

 iodide in the proportion to form mercuroso-mercuric iodide. 

 The formation of mercuroso-mercuric iodide by the action of 

 the current upon mercuric iodide may therefore be represented 



by 



(i) 2HgI 2 = Hg 2 I 2 + I 2 , 



(ii) 2HgI 2 + Hg 2 I 2 =Hg 4 I 6 ; 



or, considering it to take place in one step, 



4HgI 2 = HgJ 6 + I 2 . 



The action being slow I repeated the experiment, replacing 

 the mercuric iodide in the apparatus just described by some 

 mercuroso-mercuric iodide precipitated nearly according to the 



