486 



Prof. A. Smithells on the 



general temperature, but this added to the heat set free 

 within the flame by the decomposition of the gas does not 

 prevent solid matter separating in the flame. The acetylene 

 flame is indeed exceptionally opaque. 



I think the preceding considerations show that no reliable 

 evidence exists to warrant the supposition that the flames of 

 carbon compounds contain carbon vapour or pseudo-vapour. 

 On the contrary, there is much evidence against such a 

 supposition. 



The consideration of hydrocarbons as a likely source of 

 the Swan spectrum resolves itself into a consideration of 

 acetylene and marsh-gas, as these are the only two hydro- 

 carbons which, in presence of carbon and hydrogen, are stable 

 at high temperature (see Bone and Jerdan, Journ. Chem. 

 Soc. lxxi. p. 41, 1897). 



The fundamental difficulty presented to those who are 

 inclined to regard either acetylene or marsh-gas as the 

 source of the Swan spectrum, arises from the behaviour of 

 the flame of cyanogen. To maintain the doctrine that acety- 

 lene or marsh-gas is generated in the flame of cyanogen 

 necessitates the supposition that some impurity containing 

 hydrogen is always present. The observation of Dixon that 

 dried carbon monoxide will not burn in dried oxygen has 

 shown how important may be the part played by a minute 

 quantity of water-vapour. But in the case of cyanogen 

 experiments made by the same observer have shown that the 

 most complete dessication attainable, both of the cyanogen 

 and air, does not perceptibly affect the combustion. I have 

 dried cyanogen and air for weeks over phosphorus pentoxide, 

 but found no alteration of the spectrum of the flame, nor did 

 the deliberate addition of moisture to the gases intensify the 

 Swan component of the spectrum. 



It is impossible therefore at present to show either by an 

 absolute or differential method that the Swan bands of the 

 cyanogen spectrum are dependent on the presence of some 

 moisture which might be supposed to assist in the formation 

 of a hydrocarbon. 



If the hypothesis be maintained in the face of this fact, 

 it would necessitate the supposition that the substance to 

 which the Swan spectrum is due must have an extraordinary 

 degree of luminous efficiency. Though the facts of spectrum 

 analysis will not allow us to reject such a supposition as 

 unreasonable, it is worthy of remark that the Swan spectrum 

 is one which includes much light, and is not like that of" an 

 alkali metal, where the luminous energy is concentrated in a 

 few sharp lines. This is an argument against the luminous 



