344 Dissociation of Gases by the Electric Discharge. [May 26, 



the iodine vapour by this substance, the discharge tube was doubled 

 so that the iodine vapour was symmetrically placed with reference to 

 the sulphuric acid. The system was then placed in an oil-bath and 

 maintained at a temperature which varied in different experiments 

 from about 200° to 230°. 



On sparking through such a tube with an induction coil giving a 

 spark about 3 inches long in air, the pressure rapidly increases at 

 first, but the rate of increase gradually diminishes and the pressure 

 finally becomes steady. On stopping the coil by far the greater part 

 of this increase is permanent, or at any rate lasts for several hours. 

 It is not due to the decomposition of the vapour from the sulphuric 

 acid in the gauge, for it does not occur if there is no iodine in the 

 gauge, or if the iodine is replaced by bromine. This increase of 

 pressure can be produced by the silent discharge as well as by 

 ordinary sparking. In order to simplify the conditions as much as 

 possible, I had an arrangement made by which instead of determining 

 the increase of pressure by the sulphuric acid gauge, the vapour 

 density of the iodine after sparking could be measured. In this 

 arrangement the iodine was never near any sulphuric acid. 



The result of these determinations is shown in the following table, 

 and it is seen that the results confirm those obtained by the first 

 method. 



Unsparked iodine — 



(H =- 1). 



Pressure. Temperature. Vapour-density. 



440 215 137 



420 214 130 



Sparked iodine — 



618 220 110 



420 216 115 



166 214 84 



170 232 86 



In the last experiment the vapour- density was determined 24 hours 

 after the sparking. 



These figures point to very considerable dissociation of the iodine, 

 in fact the dissociation produced by the spark at 214° is as much as 

 that produced by Victor Meyer at the temperature 1570° C. 



The appearance of the dissociated iodine is not greatly different 

 from that of the unsparked, its colour, however, is I think a little 

 lighter and not so uniform. I was not able to detect any change in 

 the absorption spectrum produced by the sparking. The electric 

 strength of the sparked gas was however less than that of the un- 

 sparked. 



