488 REPORT— 1894. 



rounded by a furnace and kept so hot that the glass was just on the point 

 of softening ; at the end of the day the discharge tube was sealed off, 

 enough mercury having been run into a reservoir connected with it to 

 cover up the two electrodes by means of which the discharge enters and 

 leaves the tube, and also to form a large pool at the bottom, under which 

 was an electrode connected with the earth. The bulb was allowed to cool 

 to the temperature of the room, and then a discharge was sent thruugh it 

 from a high-tension transformer. The discharge presented a peculiar ap- 

 pearance, jjassing as a narrow well-defined band of light between the two 

 electrodes : it was started only after great difficulty. After the discharge 

 .stopped, the mercury vapour in the bulb was found to be positively electri- 

 fied, and this electrification had great difficulty in getting out of the gas ; for 

 although the two mercury electrodes which had been used as terminals and 

 the large pool of mercury at the bottom of the bulb were connected with the 

 earth, there was such electrification left in the bulb fifteen minutes after 

 the discharge had ceased, that when the bulb was disconnected from the 

 earth and connected with a quadrant electrometer, the spot of light re- 

 flected from the mirror was driven right off the scale. When the discharge 

 passed through mercury vapour from which other gases had not been 

 driven with so much care, there was a residual positive electrification, but 

 this disappeared so quickly after the discharge stopped, that after a 

 minute it was too slight to affect appreciably the electrometer. This ex- 

 periment shows that the communication of electricity from a gas to a 

 metal, in this case from mercury vapour to liquid mercury, is very much 

 facilitated by the presence of a third substance. 



The well-known chemical phenomenon that many chemical reactions 

 do not take place unless the temperatui-e exceeds a certain value, is also 

 paralleled by one in the discharge of electricity through gases. Certain 

 gases, such as iodine, when strongly heated, allow electricity to pass through 

 them with considerable facility ; this, however, is only possible when the 

 electrodes which carry the current into the gas are also at a high tempera- 

 ture. If we dip a piece of cold platinum foil between the electrodes when 

 the current is passing through the hot gas, the current is immediately 

 stopped apparently to as great an extent as if a piece of mica or other 

 non-conducting substance had been inserted between the electrodes ; as 

 soon, however, as the piece of platinum foil gets to a dull red heat the cur- 

 rent between the electrodes recommences, and the platinum now offers but 

 little obstacle to the passage of the current. 



The electro-motive force required to liberate the ions from an electrolyte 

 is connected in a very intimate way with the amount of work required to 

 effect the chemical change which occurs when one unit of electricity passes 

 through the electrolyte. Similarly, we may expect from the study of the 

 potential difference required to send an electric discharge through a gas, 

 to derive information about the work required to effect the chemical 

 changes which go on when the discharge passes through the gas ; as these 

 changes seem often to consist in the splitting up of molecules into atoms, 

 the study of the potential difference might be expected to throw light on 

 the amount of work required to split the molecules of a gas up into atoms. 

 Systematic measurements of the potential difference required to produce 

 discharge have been made by several observers ; they all, however, show 

 one featui-e, which, until it is investigated, makes the deduction of conclu- 

 sions as to the work required to effect the changes in the gas impossible. 

 This feature is well illustrated in an experiment described by Hittorf, its 



