assumed by Insulators at High Temperatures. 197 



through the mercury ; whilst every electrolysis which takes place 

 between two particles of mercury within the liquid conductor 

 decomposes as much periodide on the one hand as it forms on 

 the other, so that at most the position of the particles of mercury 

 will be changed. 



Of fluoride of lead, Faraday speaks still more definitely in 

 par. 1340: "When a piece of that substance, which had been 

 fused and cooled, was introduced into the circuit of a voltaic 

 battery, it stopped the current. Being heated, it acquired con- 

 ducting powers before it was visibly red-hot in daylight, and even ' 

 sparks could be taken against it whilst still solid. The current 

 alone then raised its temperature until it fused, after which it 

 seemed to conduct as well as the metallic vessel containing it. 

 During all the time there was scarcely a trace of de- 

 composing action of the fluoride, and what did occur seemed 

 referable to the air and moisture of the atmosphere, and not to 

 electrolytic action.^^ Fremy*, on the contrary, counts fluoride 

 of lead amongst the decomposable salts. 



At several different times I employed the greatest care in pre- 

 paring fluoride of lead from fluoride of potassium and acetate or 

 nitrate of lead. The electrical properties 1 found were almost 

 exactly \^hat Faraday had stated them to be, although the sub- 

 stance conducted far worse than metals, but far better than per- 

 iodide of mercury. As soon as conduction commenced, the pla- 

 tinum plates in contact with the mass became polarized. As the 

 mass began to fuse, a colourless gas escaped in small, but quite 

 perceptible, bubbles at the positive pole, around which the liquid 

 had a brown colour. A gray body collected around the negative 

 pole. The electrodes were examined after the process had con- 

 tinued an hour, during which time the galvanometer needle soon 

 went back to a position of rest. The positive electrode was 

 blackened and perceptibly acted upon. The black colour could 

 not be diminished by boiling with fuming nitric acid or with 

 potash. The negative electrode had swelled out to a porous 

 mass, in which I detected an alloy of lead and platinum ; accord- 

 ingly, the gas at the positive pole must have been fluorine, so 

 that the decomposition was quite analogous to that of chloride 

 of lead. In another experiment I found that 0*014 grm. of 

 silver had been precipitated in the voltameter, whereas the nega- 

 tive electrode had increased 0*019 grm. (instead of 0*013) ; 

 this may have arisen from an imperfect separation of the alloy 

 from the surrounding liquid, in consequence of the brittle, 

 sponge-like nature of the former. As, according to these expe- 

 riments, fluoride of lead followed the laws of electrolytic action, 

 almost exactly, I began to fear that some other compound of 



* ComptesRendus,\o\,xsyi\'m.^. 393; Journ.furprakt. CAem. vol. Ixii.p. 65. 



