Disintegration of Metals at High Temperatures. 285 



and rhodium the rate of disintegration at a given 

 temperature is roughly proportional to the oxygen 

 pressure ; for iridium, which is very oxidizable, the 

 rate of disintegration increases much more rapidly 

 with the oxygen pressure. The disintegration of 

 palladium is of a different nature and will he con- 

 sidered later. 



(c) At low pressures of oxygen the nuclei are very small. 



(d) The nuclei begin to be formed (that is, the dis- 



integration begins) at a fairly definite temperature. 



(e) The nuclei are very persistent and do not alter in size ; 



they are unaffected by light or by an electric field. 



Disintegration of Metals, 



If the hot platinum combined with the oxygen, it would 

 be expected that there would be, after some hours, a detect- 

 able diminution in the pressure of the enclosed oxygen in a 

 constant volume gas-chamber, containing a platinum wire 

 maintained at a bright heat ; and that some atomic relation 

 would exist between the loss of weight of the platinum wire 

 and the loss of weight of the oxygen in the apparatus. It 

 will be as well to mention at once, however, that the oxide is 

 unstable at temperatures lower than that at which it is 

 formed *, dissociation will be taking place, and some of the 

 oxygen will be set free. The oxygen which enters into 

 combination, and is then liberated by the dissociation, acts 

 merely as the temporary carrier of the metal. If the 

 volatile oxide could be instantly drawn away and cooled 

 very rapidly (" frozen "), practically no dissociation would 

 take place, and an atomic relation would be obtained. The 

 further the actual conditions of cooling fall short of the 

 ideal conditions of cooling, however, the further the weight 

 of oxygen lost will fall short of that required for an atomic 

 relation : until, in the limiting case, when the temperature 

 gradient is very small, all the oxide will be dissociated ; in 

 this case we may consider that the oxygen has a purely 

 catalytic effect upon the volatilization of the metal, The 

 latter conditions are approximated to in the case of the 

 platinum strip in a resistance furnace, where the temperature 

 gradient in the gas round the strip is very small. In this 

 case, as is well known, crystals of the metal are deposited in 

 the immediate vicinity of the strip, and this fact has caused 



* As the oxide is only obtained at high temperatures, it seems probable 

 that its formation is accompanied by an absorption of heat : a slight 

 lowering of temperature would therefore shift the equilibrium in the 

 direction, oxide *- metal + oxygen. 



