366 EXPERIMENTAL INVESTIGATIONS. 



converse of them ; but they were not nearly so well defined or 

 capable of being produced with the same uniformity. I have 

 endeavoured to represent one of them at fig. 7. 



I4th. In order to ascertain whether the polished ring 

 intervening between the oxidated central spot and oxidated 

 external ring were a mere negation of effect or an antithetic 

 polar effect, such as would occasion reduction, I formed in an 

 air vacuum two large spots on a silver plate, with one the plate 

 being made negative, and with the other positive, oxidating 

 them until they began to pass from deep orange to purple. I 

 then perfectly exhausted the receiver, swept it with the gas 

 employed in the last experiment, and then took the discharge 

 in a vacuum of that gas, viz. one volume oxygen + four 

 hydrogen ; the plate being positive, and the needle ^-ths of 

 an inch over the centre of each spot in turn, a ring of clear 

 polish was formed rapidly in both the dark discs, just at the 

 distance where the ring of polish appeared in the last experi- 

 ment. I then exposed a clean portion of the plate to the 

 needle without any other change, and, on allowing the dis- 

 charges to pass, formed the rings, just as in the last experiment. 



1 5th. I examined some of the spots with an achromatic 

 microscope, magnifying 200 diameters ; I could not, however, 

 discover any feature which the naked eye did not show, or 

 any peculiar molecular state ; the polishing scratches on the 

 plate were highly magnified, but the electrised spots only 

 showed more dimly the colours or the lights and shadows 

 which they exhibited to the naked eye. 



1 6th. I took the discharge on a silver plate in vacua of 

 the following gases respectively : Oxygen, protoxide of 

 nitrogen, deutoxide of nitrogen, carbonic acid, carbonic oxide, 

 and olefiant gas. 



The first four gases presented nothing remarkable ; the 

 plate was oxidated, whether positive or negative, as in a vacuum 

 of atmospheric air. In the protoxide of nitrogen the colour 

 of the discharge was a beautiful crimson on both terminals. 



In deutoxide of nitrogen a greater tendency to reduction 

 was shown when the plate was negative than in the other 

 three gases, and there was also a tendency to the formation of 

 rings. In carbonic oxide the plate was oxidated when positive, 



