of Electrolytic Decomposition. 45 



from 9° to 20° or 21°, and then fell gradually to 0° as 

 conduction ceased. The cause of this change in the resist- 

 ance is not very clear. I have assured myself that it is 

 not due to any impurity in the mercuric iodide employed, 

 nor is there a sufficient change in polarization of the electrodes 

 at the different' temperatures to account for it. With a very 

 feeble current these resistance-changes are not marked, and 

 indeed may escape observation in an experiment such as that 

 which I have described. I believe this effect to be due, first 

 to the formation, and then to the dissociation or other altera- 

 tion at the higher temperature of the mercuroso-mercuric 

 iodide produced ; in support of which it may be stated that 

 mercurous iodide undergoes rapid decomposition with separa- 

 tion of mercury at about the same temperature at which this 

 change of resistance takes place. Perhaps not entirely uncon- 

 nected with increase of resistance is the heat generated by the 

 current, in consequence of the transition-resistance at the com- 

 mon surface of the + electrode and fused mercuric iodide, 

 which may occasion the formation of vapour on the surface 

 of the electrode. I have observed this give rise to a crepi- 

 tating noise, and to the formation of a wave-motion spread- 

 ing from the + electrode over the surface of the fused 

 substance, and, under favourable circumstances, becoming 

 so marked as to throw the whole tube in which the decompo- 

 sition was being effected into violent oscillation. 



Mercuric Chloride. 



Mercuric chloride far exceeds mercuric iodide in the ease with 

 which it volatilizes at temperatures below its melting-point. It 

 melts at 265° 0. and boils at 295° C, and is more difficult to de- 

 compose with the current than mercuric iodide. This is to some 

 extent due to its higher electrical resistance, which prevents 

 the use of any complex apparatus designed to represent the 

 products of decomposition and prevent their recombination. 

 The mercuric chloride which was used w r as prepared by the 

 sublimation of the repeatedly recrystallized pure commercial 

 salt. 



The apparatus with which I have succeeded in effecting the 

 electrolytic decomposition of fused mercuric chloride is shown 

 in section in fig. 2, and consists of a glass tube of about fths of 

 an inch in diameter bent at an angle of 30°. The porous pots (ab) 

 and graphite electrodes (c d) project at each end, and the requi- 

 site temperature was supplied by the hot sand of a sand-bath. 

 With potassic iodide and starch test-paper the evolution of chlo- 

 rine at the + pole was readily detected, even when a very feeble 

 current was employed. After the current had been passed 



