Mr. Grove on the Electro-chemical Polarity of Gases. 505 



one volume oxygen mixed with four volumes of hycli-ogen, and 

 attenuated by the air-pump as in the previous experiments. 

 Tlie plate was made positive, and the point was placed success- 

 ively opposite different portions of the silver plate, at distances 

 of jV^, /oths, /oihs, 3_ths, and y^ths of an inch. The results 

 are given, as nearly as I can copy them, in the accompanvinc- 

 Plate V. figs. 1 to 5. i . o 



The colour of the central spot was a yellow-green in the centre, 

 surrounded by a blue-green, then a clear ring of polished silver, 

 then an outer ring crimson, with a slightly orange tint on the inner 

 side, and deep purple on the outer ; the exterior'portion of the spot 

 was, as far as my eye could judge, of a colour complementary to the 

 interior of the external ring, and the central portion of the spot of 

 a colour com])lementary to the exterior portion of the ring. The 

 colours varied with the time, density of gas and other conditions, 

 but generally showed this complementary tendency. Sjanptoms 

 of a faint pohshed ring were visible beyond the outer ring, and 

 could be rendered more distinct by breathing on the plate. As 

 the distance between the point and the plate was increased, the 

 colours became fainter, and the rings more diffuse, and beyond 

 the distance I have given nearly lost their defined character 3 

 but the first three distances, or those of J„th, j^ths, and /^ths 

 of an inch, gave very beautifully defined rings. The lumi- 

 nous appearance on the needle in these experiments extended 

 from three-fourths of an inch to an inch from the point. Fre- 

 quently a small polished speck was visible, exactly opposite the 

 point of the needle. See fig. 6. When the plate was made ne- 

 gative, the other conditions being the same, a polished space 

 appeared opposite the point of the needle, surrounded by a dusky 

 and ill-defined areola ; its colour, when regarded from a point 

 opposite the incident light, was brown tinged with purple ; and 

 when in the same direction as the light, a greenish white, similar 

 to the tint seen on mildew or on some of the lichens : these spots 

 were very different from the positive spots, and in some degree 

 the converse of them ; Init 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. 



14th. In order to ascertain whether the polished ring inter- 

 vening 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 tlie plate being made 

 negative, and with the other positive, oxidating them until they 

 began to ])ass from deep orange to purple. I then ])erfectly 

 exhausted the receiver, swept it with tlie gas employed in tlic last 

 experiment, and then took the discharge in a vacuum of that 



