Electric Discharge on Gases and Vapours, 523 



Without entering into detail I think it will be only neces- 

 sary to give the results of one of my experiments, which I 

 consider very successful, and which differs from the rest only 

 in better quantitative results, all being alike qualitatively. 



In the experiment alluded to, after removal of the hydrogen 

 and oxygen present in proportion necessary to form water, 

 the residues were found to consist of excess of hydrogen — 

 2'6 cubic centim. in the one tube, corresponding to the nega- 

 tive pole ; and excess of oxygen, 1*7 cubic, centim. in the 

 other, corresponding to the positive pole. 



From the arrangement of the experiment it is clear that 

 the temperatures of the electrodes must have been the same 

 and constant, i. e. 100° C. 



After having assured myself that the above differences in 

 the products of decomposition of aqueous vapour by the elec- 

 tric discharge exist, I experimented next on hydriodic-acid 

 gas, dried, and passed through the same apparatus. It was 

 found that in this instance the decomposition took place very 

 readily, and the positive pole became rapidly covered with 

 iodine, while the negative pole presented a perfectly bright 

 surface of copper. One of the products of decomposition 

 being a solid it was not attempted, of course, to collect the 

 gaseous product. On reversing the current, the poles remain- 

 ing in the same position and having been previously cleaned, 

 the same phenomenon was again observed, the iodine appear- 

 ing only on the positive pole. The temperature of sparking 

 was that of the room. But a few minutes were necessary to 

 show the deposition of iodine. By this experiment, in fact, it 

 is remarkably easy to show the distinctive action of the poles 

 in decomposing vapours to a large audience. It is only neces- 

 sary to fill a large flask having two tubulures, of the accom- 

 panying form, for example, to be 

 found in any chemical laboratory, 

 with dry hydriodic-acid vapour, 

 and then to pass the discharge for, 

 say, fifteen minutes. The elec- 

 trodes may then be removed and 

 passed round. The proof is thus convincingly furnished that 

 vapours are capable of polar decomposition. 



When the spark passes through the hydriodic-acid vapour, 

 a great part of the decomposition is of course due to thermal 

 dissociation, and a cloud of iodine vapour rises from between 

 the electrodes at each discharge, rendering this phenomenon 

 very apparent. It is hardly possible to separate perfectly the 

 two modes of decomposition that evidently have place here, 

 nor to understand how they exist side by side. 



