ELECTRO-CHEMICAL POLARITY OF GASES. 365 



marked manner than would be accounted for by the different 

 colours of the thin films of oxide formed. I determined to 

 examine this effect, and selected, after some experiments, an 

 atmosphere of one volume oxygen mixed with four volumes 

 of hydrogen, and attenuated by the air-pump, as in the pre- 

 vious 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, 5-V ns > s^ths, 

 and y^-ths of an inch. The results are given, as- nearly as I 

 can copy them, in the accompanying plate, figs. I 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 of 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 comple- 

 mentary 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 5-V tn > 

 gyhs, and ^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 t tint 

 seen on mildew or on some of the lichens : these spots were 

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



