assumed by Insulators at High Temperatures, 195 



tinued decomposition/^ Lastly, however, in par. 1341, he says, 

 " I have now very little doubt that periodide of mercury is a case 

 of the same kind^^ {i, e. a case in which a body, by fusion, 

 assumes a conducting power of its own without decomposition) . 

 In order to purify periodide of mercury, it was also twice sub- 

 limated and then placed for experiment in a glass tube, into one 

 end of which a platinum wire had been before fused. The per- 

 iodide of mercury was then retained for a long time in a state of 

 fusion, and almost of ebullition, in order to dispel any moisture 

 which might possibly have been enclosed ; the other end of the 

 tube was then closed and a second platinum wire fused into it. 

 Besides this apparatus, a galvanometer with a simple needle, and 

 a silver voltameter were introduced into the circuit ; and lastly, 

 the two platinum wires could be connected at any moment by 

 means of a commutator, with a second galvanometer containing 

 an astatic system of needles. The glass tube filled with periodide 

 of mercury was heated in a sand-bath. The salt began to con- 

 duct at about 110° before it became liquid; the commencement 

 of conduction was simultaneous with that of the yellow colour 

 which periodide always assumes immediately before fusing. 

 From the first moment of conduction, a polarization of the pla- 

 tinum wires was observed on turning the commutator, thus dis- 

 proving the hypothesis that this yellow periodide possesses con- 

 ducting power without decomposition. The glass tube was now 

 moreintenselyheated, the galvanometer needle retained atolerably 

 fixed position ; the experiment was concluded in about an hour, 

 and the glass tube broken open. Neither iodine nor mercury 

 could be detected on either wire by any reagent, although 0*005 

 grm. of silver were precipitated in the voltameter, proving that 

 about the same quantity of mercury and 0*006 grm. of iodine must 

 have been separated. Nevertheless the opinion which Faraday 

 last expressed, that periodide of mercury possesses a peculiar 

 power of conducting, could not be maintained, because the polar- 

 ization proved the existence of a permanent electrolysis. 1 hoped, 

 therefore, by a continued action to remove an electrolytic im- 

 purity which was perhaps mixed with the periodide ; and to do 

 this, I conducted a current from a zinc and iron battery of six 

 elements for fourteen hours through a new apparatus of the 

 same kind (containing about 8 grms. of iodine), taking care to 

 preserve the mass in a liquid state. The galvanometer needle 

 soon assumed a tolerably permanent position, from which it ap- 

 peared to deviate only in consequence of changes of temperature. 

 At the end of this time 0*162 grm. of silver were precipitated 

 in the voltameter -, the tube w^as opened, and even now no mer- 

 cury was perceptible on the negative wire, but in the neighbour- 

 hood of the positive wire the mass was coloured black ; and 



02 



