44 Assistant-Prof. J. W. Clark on certain Cases 



equation 



4KI + 1 2 + 2N 2 4 Hg 2 2 = Hg 4 I 6 + 4N0 2 K0. 



The precipitate was then washed, dried, fused, and finally 

 sublimed from an evaporating dish on a sand-bath into a clock- 

 glass which covered it. The substance contained a little mer- 

 curic iodide as impurity. 



The products of the electrolytic decomposition of this sub- 

 stance are mercury and iodine, of which the former is depo- 

 sited in the metallic condition, apparently according to the 

 equation 



Hg 4 I 6 =4Hg + 3I 2 ; 



whilst the iodine is absorbed by the Hg 4 T 6 in the positive 

 branch of the tube, forming mercuric iodide ; thus, 



3I 2 + 3Hg 4 I 6 =12HgI 2 . 



These results may be summarized as follows: — 

 The conduction of fused mercuric iodide is electrolytic; but 

 decomposition and recombination may take place so rapidly 

 as to give rise to an apparent metallic conduction; but my 

 investigation gives no grounds for supposing that it does not 

 quantitatively conform to Faraday f s Laws. 



The causes rendering the proof of its electrolytic decom- 

 position difficult may be summed up as follows : — 



(1) Iodine is soluble infused mercuric iodide, and so is mer- 

 curous iodide, in the latter case with the formation of Hg 4 I 6 . 



(2) Mercuric iodide is volatile, and the presence of its vapour 

 renders the detection of free iodine difficult by the ordinary test, 

 and also promotes mixture in the electrolytic apparatus by dis- 

 tillation. 



(3) It also seems possible that fused mercuric iodide possesses 

 a high diffusive rate, which would further facilitate the mixture 

 and recombination of the products of its electrolytic decomposition. 



(4 ) The electrical resistance of fused Hgl 2 is high. 



When pure mercuric iodide is fused over a gas-flame in a 

 straight glass tube, of about 6 centim. in length and 0*5 centim. 

 in breadth, and electrolyzed between platinum wire or graphite 

 electrodes with a current of about O20 ampere, the resistance 

 shows some remarkable changes. Thus: — In an experiment 

 which I copy from my laboratory journal, the resistance of the 

 mercuric iodide in a sealed tube decreased as the temperature 

 rose, until the needle of the tangent-galvanometer which was 

 included in the circuit stood at 20°; and on allowing the 

 temperature to rise still higher, it fell to 9°. During 

 cooling the inverse change occurred ; for on extinguishing 

 the gas and allowing the tube to cool ; the needle advanced 



