460 Prof. V. B. Lewes. The Cause of Luminosity [Mar. 21 r 



reached, whilst the products of combustion formed in the lower part 

 of the flame are mixed with the flame gases, partly by diffusion and 

 partly by being drawn into it by the upward rush. 



When a simple hydrocarbon like ethylene or acetylene is bnrnt 

 alone, the whole of the heat required to bring about the decomposi- 

 tion has to be generated by the combustion, without decomposition, 

 of a considerable proportion of the hydrocarbon, and this means con- 

 siderable dilution at the spot where the luminosity commences, so- 

 that at the top of the non-luminous zone of an acetylene flame there 

 is only some 14 or 15 per cent, of acetylene present, diluted with 

 nitrogen, hydrogen, water vapour, and the oxides of carbon, whilst,, 

 with a mixture of 10 per cent, acetylene and 90 per cent, of hydrogen, 

 in some cases little or no acetylene could be found at the top of the 

 inner zone of the flame, it either having diffused with the hydrogen 

 and been consumed, or polymerised to other compounds. 



It is manifest that the luminosity of a flame will be governed,, 

 not by the percentage of acetylene in the gas, but at the point at 

 which the temperature is sufficiently high to bring about decom- 

 position. 



If, instead of making a mixture of 90 per cent, hydrogen and Ifr 

 per cent, acetylene, the hydrogen is burnt at the end of an open 

 platinum tube, which has a fine platinum tube passing up the centre 

 to the top of the inner zone of the flame, and if the acetylene be 

 passed into the flame at the rate of one volume for every ten of the 

 hydrogen, not only do we obtain an intensely luminous, but a very 

 smoky flame. 



In this experiment the gases were issuing from their respective 

 tubes at the same pressure, but the small tube soon choked from 

 deposited carbon, and it was found that the same results could be- 

 equally well attained by drawing down the inner tube to the level of 

 the hydrogen tube, and making the acetylene issue at a slightly 

 higher rate of flow, which hurried it in a compact stream through 

 the inner zone of the hydrogen flame. 



In order to see if the percentage of acetylene present at the top of 

 the non- luminous zone bore any ratio to the illuminating value of the 

 mixture, experiments were made in which mixtures of hydrogen and 

 acetylene were burnt at a small flat flame burner, and the percentage 

 of acetylene was determined by gently aspirating out some of the 

 flame gases from the top of the non-luminous zone. 



Analysis of mixture used. Illuminating 



Acetylene at top of value of flame 



Hydrogen. Acetylene. non-luminous zone, for 5 cub. ft. 



65-5 34-5 372 14-0 



43-5 56-5 8-42 87'0 



0-0 100-0 14-95 240-0 



