Spectra of Carbon Compounds. 485 



lene is passed through a glass tube heated at one portion by 

 a Bunsen burner, the gas is decomposed with emission of 

 light and deposition of solid carbon. Wherever the decom- 

 posing gas is luminous there is nothing but a continuous 

 spectrum to be observed, and there is no blue base to the 

 " flame " of decomposition. 



Some estimate of the thermal result of decomposing acety- 

 lene into hydrogen and gaseous carbon may be obtained 

 indirectly. The heat of combustion of carbon monoxide to 

 carbon dioxide is 68,000 calories. No direct measurement of 

 the heat of combustion of gaseous carbon to carbon monoxide 

 can be made ; but it is highly improbable that the heat 

 equivalent for attachment of the first atom of oxygen to 

 carbon is less than for the second. From solid carbon the 

 heat of formation of carbon monoxide is 28,9G0 calories. 

 The difference between 68,000 and 28,960, namely 39,040 

 calories, may thus be taken as the minimum amount of heat 

 required for the vaporization of one atom of carbon. 



If, again, the heat of vaporization of one gramme atom of 

 carbon be calculated by Trouton's rule*, 



heat of vaporization per gramme x vapour-density __ 

 absolute temperature of boiling-point 



we have, taking 3500° as the boiling-point of carbon, and 12 

 as the vapour-density, 



■ixl2 _ 

 3500 + 273 ' 



whence x (heat of vaporization of one gramme) = 408 6, 

 and heat of vaporization of one gramme atom of carbon 

 4086 x 12 = 58,032 calories. This value is greatly in excess 

 of the one above used, and justifies the assumption of the 

 latter as a minimum value. 



Returning now to acetylene, we have for the resolution of 

 one gramme molecule into hydrogen and solid carbon an evo- 

 lution of 47,700 calories. If we subtract from this the heat 

 reckoned as above for the vaporization of 2 atoms of carbon we 

 have 47,700 — 2x39,040=— 30,380 calories ; that is to say, 

 the production of carbon vapour instead of being exothermic 

 would be highly endothermic, or, in other words, no carbon 

 vapour could be produced. 



The ordinary flame of acetylene affords the most favourable 

 condition known for the production from a hydrocarbon of 

 carbon at a very high temperature. In the outer non- 

 luminous portion of such a flame there is an extremely high- 



* Phil. Mag. xviii. p. 54 (1884). 



