228 Royal Society : — 



however, too interesting to me not to repeat the experiment ; and I 

 had six vacuum-tubes prepared, each of the same dimensions as 

 already described, two with coke balls, two with iron, and two with 

 copper, all solid : the best and most conclusive results were obtained 

 with the copper. The experiments which I now proceed to describe 

 were made on the 2 7th of last May, in the presence of the Rev. Dr. 

 Robinson and Professor Stokes. 



13. In the first tube the negative ball very soon became red-hot, 

 when suddenly the brilliant, clearly defined, conical stratified dis- 

 charge as in fig. 5 appeared, continuing for three or four seconds. 

 On breaking the circuit of the battery, we ascertained that the negative 

 ball, or that attached to the zinc terminal of the battery, had, as before, 

 entirely lost its heated appearance, while the positive, or that attached 

 to the platinum end of the battery, remained at a dull red heat, which 

 it retained for two or three seconds after the circuit of the battery had 

 been broken. 



14. A second tube with copper balls was then introduced into the 

 battery circuit ; in this I also obtained the usual luminous negative 

 glow discharge surrounding the ball attached to the zinc terminal of 

 the battery ; this glow, as before, gradually enlarged, and in a few se- 

 conds the ball became red-hot ; the circuit of the battery was then 

 instantly broken, the negative ball retaining its red heat for several 

 seconds. In this experiment it will be observed that the circuit of the 

 battery was broken before the sudden brilliant discharge, previously 

 described, had appeared. These results fully confirmed my original 

 experiment (11) of the alternate heating of the positive and negative 

 terminals of the voltaic battery, when the discharge is made in vacuo. 



15. The result of my previous experiment (11) having been thus 

 confirmed, the explanation of those that preceded it became easy of 

 solution. When both coke terminals (9) were heated to redness, the 

 continuous or arc discharge had only lasted for an instant, evolving 

 heat at the positive terminal ; but the negative during this short 

 interval retained the luminosity it had previously acquired, and con- 

 sequently at the time of the disruption of the circuit, both coke ter- 

 minal balls exhibited red heat. Again, in the first experiment with 

 the brass balls (10), the arc discharge was also only momentary, a 

 disruption of the circuit having been caused by the partial fusion of 

 the negative ball ; but the duration of this discharge was sufficient 

 to tarnish the positive terminal ; while with balls of aluminium (7), a 

 metal that fuses at a very low temperature, the negative was melted 

 by the heat evolved in the intermittent discharge ; and we ascertain 

 that under this condition the positive terminal is in no way affected, 

 for the metal retained its original lustre. 



1G. The voltaic discharge of a battery of 400 insulated cells, 

 charged with nitric and sulphuric acid, when observed in these va- 

 cuum-tubes, at first assumes precisely the same appearance as that of 

 one of nearly 4000 cells charged with rain-water, as it is also similar to 

 that of the discharge from an induction coil (fig. 2) ; but the action of 

 the nitric acid so far differs from that of the water-battery, that while 

 the discharge of the latter will remain for several weeks (or until the 



