ON GASEOUS COMBUSTION. 



4 ( J7 



that the action of the surfaces is merely an accelerating and not a 

 directive one at any particular temperature, and although this may bo 

 generally true it is not necessarily or universally so. But in regard to 

 combustion it may be assumed that, in general, the introduction ot a 

 hot surface will merely accelerate the process. 



In the generation and applications of gaseous fuels the technologist 

 has to deal not only with combustion, and the interaction of the pro- 

 ducts of incomplete combustion, but also with decomposition and 

 dissociation phenomena, and contact with hot surfaces accelerates all 

 alike. Thus the influence of hot solids— in the shape of furnace walls 

 and the like— assumes an importance which can hardly be over- 

 estimated. . , 



It will be generally agreed that the best means of elucidating trie 

 factors operative in surface combustion lies in determining the rates of 

 combination of different gases with oxygen when the reacting mixtures 

 are brought into contact with various solid surfaces at selected constant 

 temperatures. This has been the line of attack usually adopted in 

 recent investigations. But the method is only capable of yielding 

 results of any value when the temperature selected is low enough to 

 prevent the masking of the effects of surface combustion proper by 

 changes in the main body of the gas, which is not in contact with the 

 surface at any given instant. 



In interpreting the results of such velocity measurements the 

 following possible factors must be considered— namely, (1) the actual 

 rate of combination at the surface, (2) the rates at which the reacting 

 gases respectively diffuse inwards from the outside mixture on to the 

 surface, (3) the rate at which the reaction product is removed from 

 the surface, (4) the rates at which the reacting gases (or either of them) 

 are rendered ' active ' by the surface, supposing that the surface may 

 act in some such manner, and (5) changes (if any) in the physical 

 texture of the surface itself. 



It is obvious that, since any system in which a gaseous mixture is 

 combining exclusively at the surface of a heated solid must be regarded 

 as heterogeneous, the velocity of reaction will not be governed by its 

 ' order,' as would probably be the case with a homogeneous system. 

 Nevertheless several recent investigators, notably Bodenstein in his 

 earlier experiments upon the non-explosive combination of electrolytic 

 gas in contact with the walls of a glazed porcelain vessel, 1 by overlooking 

 this obvious consideration have largely invalidated their conclusions. 



Of the factors above enumerated it is now generally agreed among 

 competent observers that the actual rate of combination at the surface 

 far exceeds either the rates of diffusion of the reacting^ gases on to the 

 surface or the rates at which they are rendered ' active ' by the surface. 

 This being so, it follows that the amount of change observed in the 

 system in unit time will not be governed by the actual rate of combina- 

 tion at the surface, but by one or other of the remaining factors, which- 

 ever happens to be the slowest in its operation. 



Nernst, who has recently advanced a general theory of reactions in 

 heterogeneous systems, based on measurements of the rates of solution 

 1 Zcit. phys. Chem., 1899, 29. G6f>. 



