484 



Prof. A. Smithells on the 



sulphur vapour is not necessarily seen ; it is not seen even 

 ■when sulphur burns alone. 



" In the flame of cyanogen the carbon atoms exist for some 

 time in the free state ; the carbon bands appear in the spec- 

 trum of this flame as already remarked.'" 



The above reasoning involves, it will be seen, several 

 hypotheses. First, there is the supposition that there is an 

 interval of freedom for carbon atoms in a hydrocarbon flame 

 based, presumably, upon the idea that the oxygen selects the 

 hydrogen first. Secondly, it supposes that the liberated atoms, 

 though not part of a solid or liquid, nor yet endowed with the 

 energy necessary to constitute a gas, are capable of emitting 

 luminous vibrations. If this view were tenable we might 

 expect generally to observe spectra from chemical reactions 

 even at ordinary temperatures. 



The actual experimental evidence which we have regarding 

 the combustion of hydrocarbons and cyanogen w 7 ould lead us 

 rather to picture the process as involving primarily the 

 formation of carbon monoxide. Until the affinity of the 

 carbon and oxygen comes into play no combustion takes 

 place, and there is thus no interval for the existence of a 

 vapour or pseudo-vapour composed of carbon atoms in 

 transitu. 



Another argument has been used to countenance the 

 supposition that carbon vapour may exist in flames. It is 

 to the effect that since luminous hydrocarbon flames contain 

 solid particles of nearly pure carbon there must have been a 

 condensation of carbon vapour or pseudo-vapour from the 

 gaseous hydrocarbons. Now, according to Berthelot, the 

 separation of solid soot in flames is effected by the pro- 

 gressive coalescence of hydrocarbon molecules with elimination 

 of hydrogen, so that the passage from the gaseous hydro- 

 carbon to the solid soot is effected by a gradual enlargement 

 of the molecule and not by an abrupt condensation of carbon 

 vapour. 



Professors Liveing and Dewar after abandoning acetylene 

 as the direct cause of the Swan spectrum w r ere led to consider 

 this gas to be the means of bringing carbon vapour into 

 existence in flames. They point to the invariable presence 

 of acetylene in the interior of hydrocarbon flames and to the 

 fact that it is a highly endothermic substance. This is no 

 doubt true, but it must be remembered that the heat of forma- 

 tion of acetylene is reckoned in terms of solid carbon, and 

 there is no evidence which makes it even probable that the 

 decomposition of acetylene in a flame, assuming such to take 

 place, would yield carbon in the gaseous state. When acety- 



