oXYGKN AND ITS SALINH COMBINATIONS 



177 



flame. 1 ' The particles of charcoal burn at the outer surface of the 

 flame if the supply of air be sufficient, but if the supply of air that is, 

 of oxygen be insufficient for their combustion the flame smokes, because 

 these unconsumed particles of charcoal are carried off by the current 

 of air. 4(J 



45 If hydrogen gas be passed through a volatile liquid hydrocarbon for instance, 

 through benzene (the benzene maybe poured directly into the vessel in which hydrogen is 

 generated) then its vapour burns with the hydrogen and gives a very bright flame, 

 because the resultant particles of carbon (soot) are powerfully ignited. Benzene, or 

 platinum gauze, introduced into a hydrogen flame may be employed for illuminating 

 purposes. 



46 Inflames the separate parts may be distinguished with more or less distinctness. 

 That portion of the flame whither the combustible vapours or gases flow, is not 

 luminous because its temperature is still too low for the process of combustion to take 

 place in it. This is the space which in a candle surrounds the wick, or in a gas jet 

 is immediately above the orifice from which the gas escapes. In a candle the com- 

 bustible vapours and gases which are formed by the action of 



heat on the melted tallow or stearin, rise in the wick, and 

 are heated by the high temperature of the flame. By the 

 action of the heat, the solid or liquid substance is here, as 

 in other cases, decomposed, forming products of dry dis- 

 tillation. These products occur in the central portion of the 

 flame of a candle. The air travels to the flame from the 

 outside, and is not able to intermix with the vapours and 

 gases in all parts of the flame ; consequently, in the outer 

 portion of the flame the amount of oxygen flowing to it 

 will be greater than in the interior portions of the flames. 

 But, owing to diffusion, the oxygen, naturally together with 

 nitrogen, flowing to the combustible substance penetrates 

 inside the flame, when the combustion takes place in 

 ordinary air. The combustible vapours and gases combine 

 with this oxygen, evolve a considerable amount of heat, and 

 bring about that state of red heat which is so necessary 

 both for keeping up the combustion and also for the uses 

 to which the flame is applied. Passing from the colder 

 envelope of air to the interior of the flame, to the source of 

 the combustible vapours (for instance, the wick), we evidently 

 first traverse layers of high temperature, and then 

 layers of lower and lower temperature, in which the com- 

 bustion is less complete, owing to the limited supply of 

 oxygen. 



Thus, yet unburnt products of the decomposition of 

 organic substances occur in the interior of the flame. But flam J G ^ ie p^fon (? contains 

 there is always free hydrogen in the interior of the flame, even the vapours and products of 

 when oxygen is introduced there, or when a mixture of ^e^^he'combustionTias coni- 

 hydrogen and oxygen burns, because the temperature menced, and particles of carbon 

 evolved in the combustion of hydrogen or the carbon of are emitted : and in the pale 



zone B the combustion is corn- 

 organic matter is so high that the products of combustion pi e ted. 



are themselves partially decomposed that is, dissociated 



at this temperature. Hence, in a flame a portion of the hydrogen and of the oxygen 

 which might combine with the combustible substances must always occur in a free 

 state. If a hydrocarbon burns, and we imagine that a portion of the hydrogen occurs in 

 a free state, then a portion of the carbon must also occur in the same form in 

 VOL. I. N 



