during the passage of the Electric Discharge. 9 



is only whether no electric discharge passes through the nearest 

 possible approximation to an absolute vacuum which we can 

 procure A Torricellian vacuum, especially such a one as exists 

 in the tubes prepared by M. Cassella for Mr. Gassiot, which 

 show the broad clouds of light, is evidently more perfect than 

 the gas-vacua in the different Geissler tubes which were produced 

 by means of a mercury air-pump, and in which a lower tension 

 than 1 millim. is probably unattainable. At my request M. 

 Geissler has also lately made Torricellian vacua in longer and 

 wider tubes with great care. The best of these tubes allow the 

 passage of the direct discharge of Ruhmkorff's apparatus. This 

 discharge, which is accompanied by a white light, soon, however, 

 becomes intermittent, and after one or two minutes it completely 

 ceases. If, in accordance with the analogy of an experiment 

 before described (73), we are justified in forming an opinion as 

 to what takes place in such a tube, we must assume that the 

 oxygen of the immeasurably small quantity of air which has 

 remained behind goes to the electrode, and that the residual 

 nitrogen no longer suffices to convey the current. 



I agree with the opinion that ponderable matter is necessary 

 for the formation of an electric cuiTent. Such matter is, how- 

 ever, in general a gas, and not as (at least partly) in Davy's lumi- 

 nous arc, metal or carbon passing over in the extremest state of 

 division*. 



93. I introduce here an observation which was made with 

 every tube containing a Torricellian vacuum. Coatings of tin- 

 foil were fastened near to both ends of such vacua, in order to 

 be able to pass induced currents through them. (See subsequent 

 par. 118). After the passage of the dn-ect current had ceased, 

 the induced current might still be passed through for a long- 

 time with undiminished brightness. On reconnecting the wire 

 extremities of Ruhmkorff's apparatus with the ends of the two 

 platinum wires fused into the ends of the tubes, the direct cur- 

 rent was re-established, but soon disappeared again. The same 

 experiment might be repeated at pleasure. 



94. Most gases, when the current is passed through them in 



* The transference of the substance of the electrodes from the one to 

 the other of them has been, since Davy's time, the subject of numerous 

 investigations. In general it was assumed that this transference took place 

 from the positive to the negative electrode. M. von Breda, however, 

 proved that a simultaneous transference occurred from the negative to the 

 positive. In Geissler's tube the particles of metal simply leave the nega- 

 tive electrode without reaching the positive one. It would seem as if 

 anomalies existed here similar to those relating to the occurrence of heat 

 at both electrodes (4()). Experiments which t have quite recently per- 

 formed upon the occurrence of the luminous arc in bulbs of rarefied gases 

 (under the influence of the nuignet), anil whicli will be described iu the 

 next communication, will, I believe, throw light upon the subject in question. 



