surrounding Media on Voltaic Ignition. 125 



Upon the whole, we may conclude, from the experiments 

 detailed in this paper, that the cooling effect of different gases, 

 or rather the difference in the cooling effect of hydrogen and 

 its compounds from that of other gases, is not due to differences 

 of specific heat ; it is not due to differences of specific gravity ; 

 it is not due to differences of conducting powers for electricity ; 

 it is not due to the character of hydrogen in relation to its trans- 

 mission of sound, noticed by Leslie, for reasons which I have 

 before given*; it is not due to the same physical characters 

 of mobility which occasion one gas to escape from a small 

 aperture with greater facility than another; but it may be, 

 and probably is, affected by the mobile or vibratory character 

 of the particles by which heat is more rapidly abstracted. I 

 at one time thought that the effect might have relation to the 

 combustible character of the gas, and that the electro-negative 

 gases were in respect to it contra-distinguished from the elec- 

 tro-positive or neutral gases, but the experience I have ob- 

 tained from the experiments detailed here induces me to 

 abandon that supposition. 



I incline to think, that, although influenced by the fluency 

 of the gas, the phaenomenon is mainly due to a molecular 

 action 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 radia- 

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

 every one knows, strikingly contra-distinguished in this re- 

 spect: why may not the surface of the gaseous medium conti- 

 guous to the radiating substance exercise a reciprocal influ- 

 ence ? 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 esta- 

 blish a link of continuity between the cooling effects of differ- 

 ent 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 further development of them 

 appears to me the most probable guide to the connexion by 

 definite conceptions of physical and chemical actions. 



The difference of the cooling effect of hydrogen, and of 

 those of its compounds, where it is not neutralized by a pow- 

 erful electro-negative gas, from all other gases, is perhaps the 

 most striking peculiarity of the phaenomena I have described. 



* Philosophical Transactions, 1847. [Phil. Mag. S. 3. vol. xxxi. p. 22.] 



