MR. GROVE ON THE ELECTRO-CHEMICAL POLARITY OF GASES. 
93 
after some experiments, an atmosphere of one volume oxygen mixed with four volumes 
of hydrogen, and attenuated by the air-pump as in the previous experiments. The 
plate was made positive, and the point was placed successively opposite different 
portions of the silver plate, at distances of -^th, s^ths, :^ths, -^oths and ^ths of an 
inch. The results are given, as nearly as I can copy them, in the accompanying 
Plate, figs. 1 to 5. 
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- 
plementary 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. Symptoms of a faint polished 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 ; but the first three distances, or those of -^th, s^ths and 5 %ths of an 
inch, gave very beautifully defined rings. The luminous appearance on the needle 
in these experiments extended from three-fourths of an inch to an inch from the 
point. Frequently a small polished speck was visible, exactly opposite the point of 
the needle. See fig. 6. When the plate was made negative, 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 ; but they were not nearly so well defined or capable of being 
produced with the same uniformity. 1 have endeavoured to represent one of them 
at fig. 7. 
14th. 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-j-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 experiment. I then exposed a clean portion of the plate to the needle without 
