WHY A FLAME EMITS LIGHT. 



215 



II 



1861 the view, it seems, was that carbon is liberated because of a sup- 

 posed greater affinity of oxygen for the hydrogen of the hydrocarbon 

 than for the carbon, there not being enough for both. But these points 

 had to be tested. 



In the study of the chemical changes that take place, a flame burn- 

 ing at a circular orifice offered the best conditions. As explained in 

 text-books of chemistry, such a flame may be thought of as being made 

 up of an inner, faintly luminous cone fitting into an outer, brightly 

 luminous one — as a finger fits into a glove finger — this latter being 

 surrounded by a non-luminous sheath of water vapor and carbon diox- 

 ide. It was desirable to separate these two cones, in order to study 

 the gas after it had left the inner cone and before any change had been 

 brought about by the conditions existing in the outer 

 cone. This separation was first accomplished by Techlu, 

 in France, and Arthur Smithells, in England, work- 

 ing independently, with a piece of apparatus, the 

 essential features of which are pictured in cross-sec- 

 tion in Fig. 1. By a proper control of the relative 

 proportions of gas and air the inner cone was made to 

 burn at the orifice i, while the outer cone burned at 

 the orifice o. The outer cone got its oxygen from the 

 surrounding air, while that for the lower flame was sup- 

 plied along with the gas. The temperature of each 

 cone was measured and the gases entering and leaving 

 each were analyzed. It was found that as the propor- 

 tion of gas to air was increased, the tip of the inner or 

 lower cone became brightly luminous and a column of 

 soot passed upward through the tube, becoming faintly 

 luminous in the outer edge of the upper flame. As soon 

 as the inner cone becomes luminous the unsaturated* 

 hydrocarbon compound known as acetylene begins to 

 appear among the gases passing to the outer cone. 



Vivian B. Lewes now attacked the problem as to how carbon comes 

 to be in the flame in the free state. He analyzed gas drawn from dif- 

 ferent parts of a coal-gas flame, measured the temperature of its dif- 

 ferent parts, etc., publishing his results between 1893 and 1895. These 

 results may be stated as follows : Coal-gas consists mainly of a mixture 

 of hydrogen and hydrocarbons, both saturated and unsaturated. In an 

 ordinary 'fishtail' burner flame all hydrogen is consumed before the 

 middle of the luminous portion is reached. Of the saturated hydro- 

 carbons about seventy-five per cent, disappears as such in the dark por- 



* The terms ' saturated ' and ' unsaturated ' have reference, amonsr other 

 things, to the relative quantity of hydrogen to carbon in the molecule, an un- 

 saturated compound having relatively less hydrogen than a saturated one. 



k. 



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Tic. I. 



