of Electricity through Hot Gases. 363 



tube the terminals insulated in the way just described dipped. 

 The platinum tube was wrapped round with asbestos tape, 

 then placed in an iron tube, and the whole arrangement put 

 into one of Fletcher's muffle-furnaces, fed by 1-inch gas-pipe 

 and a large air-blast worked by bellows such as are ordinarily 

 used for forges. 



If the substance experimented on was solid or liquid at 

 ordinary temperatures, the gas was produced by dropping the 

 solid or liquid into the tube and allowing the vapour from 

 it to drive the air out ; if the substance was gaseous at ordi- 

 nary temperatures, a stream of it was conducted through 

 the stem of a clay tobacco-pipe to the bottom of the tube 

 and allowed to run until it had driven the air out. 



The method of procedure was very simple. The electrodes 

 in the tube were placed in series with the galvanometer and 

 with a large DanielFs battery arranged in boxes with twelve 

 cells in each ; a reversing-key was placed in the circuit. When 

 the gas had reached the requisite temperature, the supply of gas 

 to the furnace was stopped, the battery put on one way, and 

 the deflexion of the galvanometer read ; the battery was then 

 reversed and the deflexion again read. Though the deflexions 

 are given in some cases the results must be considered as 

 qualitative rather than quantitative, as the temperature was 

 only fixed by describing the glow from the tube as dull red, 

 bright red, yellow, bright yellow, and white. 



It was found as soon as different gases were tested that 

 they differed enormously in their power of acquiring con- 

 ductivity by heating. Some of them, such as air or nitrogen, 

 only acquired it to a very small extent; and I shall afterwards 

 give reasons for believing that most of this can be accounted 

 for by convection-currents produced by the disintegration of 

 the electrodes. In other cases, however, notably for hydriodic 

 acid gas, iodine vapour, bromine vapour, the vapour of sodium 

 chloride, hydrochloric acid gas, the vapours of sodium and 

 potassium, the gas acquired very considerable conductivity 

 when the temperature exceeded a red light; so much so in fact 

 that at a white heat two or three Leclanche cells were sufficient 

 to throw the light reflected from the galvanometer-mirror 

 right off the scale. The difference between the conductivity 

 for the two classes of gases is so great that it suggests that 

 the method by which the electricity is conveyed from one 

 electrode to the other is not the same in the two cases. 



It ought to be mentioned that even in the case of those 

 gases which conduct best, there was not sufficient conductivity 

 to produce a current that could be detected by the galvano- 

 meter until the electrodes began to glow; and from <ome 



