ON GASEOUS COMBUSTION. 489 



several well-established cases in which combustion apparently does not 

 depend upon the presence of moisture. 



4. In 1893 Sir J. J. Thomson ' pointed out that if the forces holding 

 the atoms together in a molecule are electrical in character, the presence 

 of drops of any liquid (such as water) of high specific inductive capacity 

 would probably cause a sufficient loosening of the bands between the 

 atoms to render the molecule much more reactive. He showed that 

 the complete drying of a gas renders it non-conductive. H. B. Baker, 

 in his Wilde Lecture before the Manchester Literary and Philosophical 

 Society, 2 described a number of new experiments which led him to 

 put forward tentatively the theory that chemical interchanges in gaseous 

 systems depend upon the presence of both ' ions ' and water vapour; 

 the ' ions ' act as nuclei for the condensation of steam, and the liquid 

 drops of water so formed, by virtue of their high specific inductive 

 capacity, facilitate chemical change in the layer of gas immediately in 

 contact with them. Chemists will await with the greatest interest tbe 

 further development of this hypothesis, but the idea that such rapid 

 changes as are met with in gaseous explosions are dependent upon the 

 formation of aggregates of steam molecules in an atmosphere containing 

 fewer than four of them per 1,000 millions, and that such aggregates 

 approximate to liquid drops at the high temperatures of flames, makes 

 large demands upon the imagination, and it will require to be supported 

 by the strongest experimental evidence. 



Section V. — The Combustion of Hydrocarbons. 



The question of how a hydrocarbon is attacked by oxygen in com- 

 bustion has been the subject of much controversy during the past 

 twenty years, but it is only quite recently that experimental inquiry 

 has been pushed far enough to justify the advancement of any complete 

 theory of the process. 



Throughout the greater part of last century it was accepted as an 

 article of faith among chemists that hydrogen is the more combustible 

 element of a hydrocarbon; thus, as late as 1884 H. B. Dixon, in 

 his Cantor lectures on ' The Use of Coal Gas,' speaking of the com- 

 bustion of ethylene in its bearing on the luminosity of hydrocarbon 

 flames, said: ' This ethylene, when it is raised to a high temperature in 

 contact with air, is decomposed, the hijdrogen burning first and the 

 carbon afterwards. There is a race for the oxygen of the air between 

 the two constituents of the ethylene, and the hydrogen, being the fleeter 

 of the two, gets to the oxygen first, and is burnt to water.' Eight years 

 later, when it was discovered in Dixon's laboratory that an equi- 

 molecular mixture of ethylene and oxygen yields on detonation almost 

 exactly twice its own volume of carbon monoxide and hydrogen, 3 in 

 accordance with the empirical equation 



C 2 H 4 +0 2 =2CO + 2H 2> 



' Phil. Mag., 36, 321. 2 Man. Mem., 53, part iii. 



' H. B. Dixon, Phil. Trans., 1893, 159 ; also Lean and Bone, Trans. Ghem Soc 

 1892, 61, 873. 



