Hydrogen with Oxygen at Low Pressures. 475 



round the wire, and must, almost as soon as it was formed, 

 have diffused to the pentoxide of phosphorus which was 

 contained in the apparatus, had a large surface, and lay 

 under the whole length of the wire (fig. 1). 



The phenomenon is more probably due to another cause. 

 It has been pointed out that some discontinuity in the state 

 of the platinum must be reached when the chemical com- 

 bination sets in. The effect of this probably persists for a 

 time : probably the platinum does not immediately recover 

 its former condition, and only slowly returns to that condition. 

 Such a hysteresis as this is not uncommon in molecular 

 phenomena. But a more precise conjecture may be hazarded. 

 The chemical reaction observed when the platinum attains a 

 definite temperature has been explained by the assumption 

 that a sudden increase takes place in the corpuscular discharge 

 from the platinum at the moment when the platinum reaches 

 that temperature. Is it not likely that such an eruption 

 would temporarily modify the surface of the platinum, and 

 thus make a similar eruption possible at a temperature rather 

 lower than before ? 



The experiments recorded in this paper were not carried 

 out under idea] conditions of purity ; and indeed under such 

 conditions they would have been impossible. Nevertheless, 

 every reasonable precaution against impurities was taken. 

 The electrolyte (barium hydrate) insured the purity of the 

 hydrogen and oxygen. The platinum wires and the tube 

 which carried them were thoroughly cleaned, and the whole 

 apparatus was made thoroughly airtight. The mixed 

 hydrogen and oxygen never, of course, became wholly 

 free from air. But the proportion of air in the mixture 

 was always very small and steadily diminished ; and the 

 consistency which, while this diminution was in progress, 

 continued to characterize the observations shows that the 

 results are the same as if the air had been wholly eliminated. 



Summary. 



The general conclusions to be drawn from the experiments 

 described or alluded to in this paper may now be summarized. 



Platinum in the presence of hydrogen and oxygen at a low 

 pressure does not cause these gases to combine perceptibly 

 unless its temperature exceeds a certain critical value. 



That critical value is approximately independent of the 

 pressure of the mixed hydrogen and oxygen, so long at least 

 as the pressure lies between 40 mm. and 2 mm., and is 

 about 275° centigrade for pure platinum. 



