Disintegration of Metals at High Temperatures. 295 



and under these circumstances, truly, the influence of the 

 oxygen is catalytic. 



We see that this theory of the formation of an endo- 

 thermic, volatile, and dissociable oxide explains all the 

 observed facts in a perfectly simple manner and without 

 any other assumptions. 



The formation of an oxide of platinum would also explain 

 certain facts in the discharge of negative and positive 

 electricity from hot platinum wires at low pressures. It is 

 well known that a metal plate placed in the neighbourhood 

 of a hot platinum wire acquires an electrical charge. "If 

 the wire be not too hot, then at high pressures the plate 

 will be charged positively ; on exhausting the vessel, a 

 point will be reached where the positive charge begins to 

 decrease, then vanishes, and finally is replaced by a negative 

 charge. This change in the sign of the charge on the plate 

 occurs at much higher pressures in hydrogen and nitrogen 

 than in oxygen, where this reversal is difficult to obtain 

 unless the wire be very hot. When the reversal of sign has 

 been obtained in hydrogen or nitrogen, the addition of a 

 surprisingly small quantity of oxygen is sufficient to make 

 the charge on the plate positive again. It is possible that 

 part of the diminution in the positive leak may be due to 

 the burning up of the oxygen. The increase in the positive 

 electrification produced by oxygen is easily explained if 

 there is any oxidation of the metal at a red heat ; for in the 

 oxide thus formed, the oxygen carries the negative, the 

 metal the positive charge : thus if the oxygen in the neigh- 

 bourhood of the platinum wire got ionized by the heat, the 

 platinum, by combining with the negative but not with 

 the positive oxygen ions, would leave an excess of positive 

 ions in the neighbourhood " *. There are many other 

 observations in the discharge of electricity from hot metal 

 wires which would receive their most satisfactory explanation 

 on the theory of the formation of an unstable oxide f- 



The results of these experiments have obvious bearings 

 upon the means to be adopted for the preservation of the 

 various metallic heating strips used in resistance furnaces. 



I desire to offer my very best thanks to Prof. Donnan, 

 F.R.S., and to Mr. R. E. Slade, M.Sc, for their kind 



* J. J. Thomson, l Conduction of Electricity through Gases/ 1903, 

 p. 181. 



t See various parts of Ch. 2-5 of ' Electrical Properties of Flames 

 and Incandescent Solids,' by H. A. "Wilson. University of London. 

 Press, 1912. 



