during the passage of the Electric Discharge. 1 1 



touching the dark part of the tube near the negative electrode 

 with a weak horseshoe magnet, the system of white clouds were 

 drawn further into the dark space : a beautiful appearance was 

 thus presented, which I had already seen produced by Mr. Gas- 

 siot, but which I had up to that time not been able to produce 

 in Geissler^s tubes. On touching with two fingers that part 

 of the tube where the first light-cloud bordered on the dark 

 space, the whole of the clouds were driven back towards the 

 positive pole ; that is, there was repulsion where in the former 

 case attraction occurred, and the whole phsenomenon became 

 more stable. In this case the tube presented the appearance of 



95. The same experiment succeeded with all similar tubes ; but 

 they did not all present quite the same appearance after the change 

 had taken place. In place of the inch-wide white cloud, there 

 frequently occurred a much narrower stratification of the white 

 light, such as always occurs in the Torricellian vacuum. In one 

 instance, indeed, the light was of a reddish colour, correspond- 

 ing to that produced by nitrogen, and of the tint which I ob- 

 served it gradually to become in the hands of Mr. Gassiot when 

 he, by an ingenious contrivance, allowed traces of air to enter 

 very gradually. It hence appears certain that in all the " sul- 

 phurous-acid vacua," greater or less traces of air were present, 

 and that this was especially the case in the tube last described. 



It is probable that sulphurous acid, when quite dry and free 

 from air, furnishes us with the best means for making the best 

 approximation to an absolute vacuum. 



96. The following observation seems to me worthy of notice. 

 On heating the extremities of the cyhudrical tube into which the 

 platinum electrodes were fused, over a spirit-lamp, the original 

 beautiful stratification in the violet light immediately reappeared. 

 The sulphurous acid was formed again by the recombination of 

 its constituents. The latter (probably both of them) had become 

 combined loosely with the platinum, and were separated from it 

 again by the heat of the lamp. The passage of the current de- 

 termined the re-formation of the white cloudy stratification by 

 gradually decomposing the sulphurous acid. After this stra- 

 tification had again ceased, it might be again restored by a 

 fresh application of heat ; and thus the two phsenomena might 

 be alternately produced a few times, but each time with dimi- 

 nished beauty. 



97. It seems to me to follow from the above, that the light 

 accompanying discharge through tubes which have been filled 

 with any gas, and exhausted so as to contain only traces of the 

 gas, is quite independent of the metal of the electrode, and, 

 further, that no discharge takes place through an absolute 



