444 Prof. J.J. Thomson on the Passage 



this effect, as before the hot gas can be cooled it must come 

 into contact with the electrode ; and as there is a strong up 

 current of hot gas fresh supplies of it are continually coming 

 into contact with the electrode ; so that if the hot gas could 

 give up its electricity to the cold electrode there would be a 

 current through the gas. The difficulty with which cold 

 metals get electrified by hot gases is also shown by the 

 following experiment : — two platinum electrodes were im- 

 mersed in iodine at a yellow heat, and a piece of platinum 

 foil, whether insulated or uninsulated is immaterial, was 

 arranged so that it could be lowered between the electrodes 

 or raised from the hot gas at pleasure. Initially the platinum 

 foil was out of the iodine and a current was flowing through 

 the hot gas ; the foil was then lowered into the hot gas and 

 at once stopped the current. As soon, however, as the foil 

 began to glow the current recommenced, and the system finally 

 conducted as well as before the introduction of the foil. These 

 experiments seem to show that for electricity to get from the 

 gas into the electrode, or in other words for the atoms to give 

 up their charges to the electrodes, the latter must be at least 

 red hot. Whether this is due to the disintegration of the 

 electrodes at a red heat, or to the increase in the temperature 

 of the electrode producing an increase in that action between 

 the gas and the metal which manifests itself in the case of 

 some gases and metals as chemical action, is a point which I 

 hope to investigate more fully. 



Influence of the Material of the Electrode. 



Electrodes of platinum, iron, carbon, gold, and aluminium 

 were tried ; the experiments with the aluminium electrodes 

 were not, however, satisfactory, as the electrodes melted as 

 soon as the temperature was high enough to make the gas 

 conduct. The resistance of a conducting gas, such as iodine, 

 seemed practically the same whichever of the first four sub- 

 stances were used. Copper electrodes seemed also to behave 

 in the same way until they got covered with a coating of 

 oxide. 



Symmetry of the Conduction. 



There does not seem to be in the conduction of electricity 

 through those hot gases which conduct the most readily, that 

 dissimilarity in the properties of the positive and negative 

 electrodes which is found in the discharge through vacuum 

 tubes and in the more closely analogous case of conduction 

 through flames. This was tested by using two electrodes, A 

 and B, which were very unlike in shape and size: in most of the 

 experiments A was a piece of platinum foil, while B was a thin 

 platinum wire. The deflexion of the galvanometer was observed, 



