VOLTAIC IGNITION. 349 



at the surfaces of the ignited body and of the gas. We know 

 that in the recognised effects of radiant heat the physical state 

 of the surface of the radiating or absorbing body exercises a 

 most important influence on the relative velocities of radiation 

 or absorption ; thus, black and white surfaces are, as every 

 one knows, strikingly contra-distinguished in this respect : 

 why may not the surface of the gaseous medium contiguous 

 to the radiating substance exercise a reciprocal influence ? 

 Why may not the surface of hydrogen be as black and that of 

 nitrogen as white to the ignited wire ? This notion seems 

 to me the more worthy of consideration, as it may establish 

 a link of continuity between the cooling effects of different 

 gaseous media and the mysterious effects of surface in catalytic 

 combinations and decompositions by solids, such as platinum. 

 Epipolic actions will, I feel convinced, gradually assume a 

 much more important place in physics than they have hitherto 

 done ; and the farther development of them appears to me 

 the most probable guide to the connection by definite con- 

 ceptions of physical and chemical actions.* 



The difference of the cooling effect of hydrogen, and of 

 those of its compounds, where it is not neutralised by a 

 powerful electro-negative gas, from all other gases, is perhaps 

 the most striking peculiarity of the phenomena I have de- 

 scribed. The differences of effect of all gases other than 

 hydrogen and such compounds are quite insignificant when 

 compared with the differences between the hydrogenous and 

 the other gases. There are some phenomena which I have 

 before observed, and which were, at the time I noticed them, 

 inexplicable to me ; but they now appear dependent on this 

 physical peculiarity of hydrogen. Thus, if a jet of oxygen 

 gas be kindled in an atmosphere of carburetted hydrogen, the 

 flame is smaller than when the converse effect takes place. 

 The voltaic arc between metallic terminals is also much 

 smaller in hydrogen gas than in nitrogen, though both these 

 gases are incapable of combining with the terminals ; indeed, 

 to obtain an arc at all in hydrogen is scarcely practicable. 



Davy has, in his ' Researches on Flame,' given several 



* See Dr. Tyndall's ' Researches on the Absorption and Radiation of Heat 

 by Gaseous Matter.' /V7. Trans. t 1861, 1862, and 1864. W.R.G., 1874. 



