of Electricity through Hot Gases. 365 



hydrochloric acid at high temperatures, and Scott's determina- 

 tion of the vapour-density of KI, show that there cannot be 

 any very great amount of dissociation for these substances. 

 I thought, however, it would be interesting to test whether 

 or not there was any dissociation of the KI and HC1, since they 

 conducted electricity when raised to a red heat. The po- 

 tassium iodide was tried first. The stem of a long clay tobacco- 

 pipe was placed in the platinum tube, one end reaching down 

 to the bottom (the hottest part) of the tube, the other end 

 connected by a glass tube with a bulb filled with a solution of 

 starch; another glass tube was fixed on to this bulb and con- 

 nected with a water-pump, so that when the pump was 

 working, the gas from the bottom of the platinum tube bubbled 

 rapidly through the starch solution: it thus passed very rapidly 

 from a very hot to a cold place, so that if any dissociation 

 took place in the platinum tube the constituents might not have 

 time to combine before passing through the solution, where 

 the presence of free iodine would be detected. On trying the 

 experiment, a very distinct blue coloration of the solution 

 was obtained, though no such coloration was produced by 

 driving hot air through the bulb just before the KI was put 

 in the platinum tube. The experiment was repeated several 

 times, always with the same results; and we may therefore 

 conclude that free iodine was present, in other words, in the 

 KI which conducted the electricity there was dissociation. 



A similar experiment was then tried with the hydrochloric- 

 acid gas, the only difference being that the bulb now contained 

 a solution of starch and iodide of potassium instead of a 

 solution of starch alone. If the HC1 is dissociated into H and 

 CI, and if these gases can be withdrawn from the hot platinum 

 tube so quickly that they have not time to recombine, on 

 bubbling through the solution the free chlorine will set free 

 iodine from the potassium iodide and produce a blue colora- 

 tion of the solution. On trying the experiment, the solution 

 began to change colour as soon as the gas commenced to 

 bubble through, and in less than a minute became almost 

 black. This was not due to any oxide of nitrogen, because 

 the coloration did not take place when air from the bottom 

 of the hot platinum-tube was sucked through the solution, 

 nor was it due to any impurity in the hydrochloric acid, for 

 it did not occur when the HC1 was allowed to bubble through 

 the solution without being heated. 



A similar experiment was tried withNaCl, and free chlorine 

 was found in the hot vapour of this gas, though the effects 

 produced by it were not so large as in HOI, where the solution 



