of Electrolytic Decomposition. 41 



edges of the glass vessel in which fusion and sublimation are 

 effected, may decompose, yielding free iodine vapour. Pure 

 mercuric iodide may be fused and sublimed without under- 

 going any such change ; nor have I any reason to believe that, 

 when strongly .heated in a sealed glass tube, iodine is ever set 

 free, a slight darkening in the colour of the fased substance 

 being the only apparent alteration which it then undergoes. 



Here, too, perhaps it may be convenient to describe the 

 preparation of the graphite electrodes, which for some years 

 past I have found very convenient for the decomposition of 

 such substances as act upon platinum. These electrodes are 

 best made from the " leads " of Rowney's HH cedar pencils, 

 which may be easily removed after a few hours' soaking in 

 water has softened the glue sufficiently to allow of the pencil 

 being split in half. The " lead " is then removed with a knife, 

 and only requires heating to bright redness in a Bunsen- 

 flame (to get rid of the shellac (?) which it contains) to render 

 it fit for use. A platinum wire twisted or bound round one 

 end makes a good connection for the battery-terminals. 



When pure mercuric iodide in a sealed glass tube is kept 

 in a state of fusion over a gas-flame, and electrolyzed by 

 means of two platinum-wire electrodes passing through its 

 ends, evidence of its decomposition may be obtained from the 

 iodine set free about the + pole, although no mercury is dis- 

 coverable at the negative. The quantity of iodine thus set 

 free is, however, small, and does not usually seem to increase 

 with the length of time that the current is allowed to pass ; 

 whilst the decomposition, judging from the liberation of iodine 

 vapour about the positive or upper electrode in the tube, 

 appears to take place at temperatures very little above the 

 solidifying-point of the liquid mercuric iodide. 



Electrolysis of this substance kept fused in narrow V- and 

 W-shaped tubes over a gas-flame yields much the same results; 

 but in such tubes the resistance is very high, and it is more- 

 over difficult to ascertain precisely what is taking place within 

 them. I therefore adopted a simple V-shaped glass tube of |ths 

 of an inch diameter and bent at an angle of about 30°, into 

 which was placed a sufficient quantity of mercuric iodide for 

 the experiment. The substance was then kept fused over a 

 gas-flame, and the two graphite electrodes introduced, one at 

 each end of the tube. In such a tube the behaviour of the 

 substance is easily observed ; and by blowing air dried over 

 chloride of calcium into the end, the issuing vapours are readily 

 tested for iodine with starch-paper. Under these conditions, 

 I found that when the substance was at a temperature near 

 the melting-point iodine could usually be detected with ease, 

 although at a higher temperature none could be shown to 



