400 Dr. Richardson: Effect of a Luminous Discharge 
corresponding cathode-fall; in the lower, the anode-fall 
multiplied by 10. 
Our hypothesis appears to be supported throughout by the 
experimental results. The electrode fall may be represented 
as a continuous function of the contact potential of the 
electrode. The curves show some interesting exceptions. 
Platinum exhibits a slight tendency to drop below its normal 
value as cathode, but is in line as anode. Gold appears in 
general to be displaced in the electrode series to the neigh- 
bourhood of tin and lead. This is ascribed to the effect of 
mercury-vapour in that the surface of the gold always bleached 
while standing én vacuo. Gold-foil interposed between the 
discharge-tube and the parts containing mercury did not 
prevent this bleaching effect. Being heated to redness, the 
metal regained its natural tint. 
In nitrogen, cadmium and zine are displaced to the platinum 
end in the electrode series, the cathode-fall, as already noted, 
being for these metals abnormally high, while the anode-fall 
is abnormally low. 
In oxygen, copper drops as cathode slightly below its 
normal position ; but steel and nickel form interesting excep- 
tions in being displaced to the vicinity of zinc in the electrode 
series. A similar inconsistency is exhibited by these metals 
when the potential series in sulphuric acid and that given by 
the heat of combination of the metals with it are compared *. 
In conclusion, it should be remarked that, as shown by 
these curves, the electrode fall is not a constantly increasing 
quantity in passing from one end of the potential series to the 
other; but that, starting with a minimum value at one end, 
it approaches at a decreasing rate a constant maximum value 
for the rest of the series. This is clearly marked for all curves 
except that giving the anode-fall in nitrogen. 
April, 1904. 
XXXIX. The Effect of a Luminous Discharge on the Ioniza- 
tion produced by Hot Platinum in Gases ut Low Pressures. 
By 0. W. Ricwarpson, M.A., D.Sc., Fellow of Trinity 
College and Clerk Maxwell Student in the University of 
Cambridget. 
PJXHE present communication forms the continuation of an 
investigation the results of which were embodied in a 
paper read before the Physical Society on June 12, 1903, and 
published in the Philosophical Magazine for the following 
* Jahn’s Grundriss d, Elektrochemie, p. 7. 
7 Communicated by the Author. 
