42 



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



FiR. 1. 



exist in the free state. A control experiment with fused 

 chloride of lead, using a current of the same strength and the 

 same distance between the electrodes, gave evidence of chlorine 

 without difficulty. 



Hence it appeared possible that this substance conducted 

 (as Ag 2 S had been supposed to do) in two ways — (a) electro- 

 lytically, at a temperature at which it just became liquid; and 

 (/3) metallically, at higher temperatures. But during some 

 observations upon the expansion of fused mercuric iodide in a 

 thermometer-like tube, I noticed that the liquid underwent 

 great contraction during cooling from a temperature a little 

 above the melting-point, accompanied by a distinct loss of 

 fluidity. To the latter change I at once attributed the evi- 

 dence of electrolytic decomposition, which I had observed at 

 a corresponding temperature, to be probably due, as it would 

 be less favourable to the mixing and recombination of the 

 products of electrolytic decomposition; and I therefore turned 

 my attention to the construction of an apparatus in which 

 they should be so separated as to render this less easy. 



The form of apparatus which I finally adopted is shown in 

 section in fig. 1. The tube (a b c) 

 containing two small porous bat- 

 tery-pots (gf), the graphite elec- 

 trode, and the mercuric iodide to 

 be electrolyzed, is U-shaped, and 

 the two branches are connected 

 by a constricted portion (c), which 

 further materially hinders the 

 mixing of the fused products of_ 

 decomposition in the two branches 7 

 of the tube. This apparatus 

 passes through a hole in the tin 

 cover (Ji) which su ports it, and 

 dips into the oil in the brake 1 ", 

 which is heated to the desired 

 temperature by means of a gas- 

 flame or sand-bath, at which it is 

 kept constant by placing the 

 bulb of the air-thermometer (It) 

 in communication with a gas- 

 regulator. When mercuric iodide 

 is electrolyzed in such an appa- 

 ratus with a current of about 0*02 



J.W.CLARK DEL 



ampere, iodine is liberated at 



temperatures far above the melting-point of the substance. 



If the current v as too strong the circuit was usually 



