380 THE POPULAR SCIENCE MONTHLY. 



produced in the fire, and not that of the coal itself. Only gases burn 

 with aflame. In those cases where a solid or liquid seems to do so, it 

 will he found that it is either volatilized or decomposed by the heat of 

 the combustion, and thus converted into gas before it burns. 



Let us now begin our study of the phenomena of flame by consid- 

 ering the flame of hydrogen. Suppose we have hydrogen-gas flowing 

 from a round jet. Just over the opening we have a round column of 

 pure hydrogen. This gas, being lighter than air, and being forced out 

 under some pressure, rises. As it rises it mixes with the air, and we 

 immediately have, surrounding the jet of hydrogen, a layer of mixed 

 hydrogen and air which is inflammable. If we now apply a light this 

 mixture takes fire. The hydrogen unites with the oxygen of the air, 

 forming steam, which is carried away by the current of hot gases ; 

 more hydrogen is continually supplied from the jet, and more oxygen 

 from the atmosphere : and thus we have a continuous formation, as 

 fast as it is burned, of this inflammable mixture of hydrogen and oxy- 

 gen around the central column of hydrogen. 



Evidently, then, the flame must be hollow. That it is so may be 

 shown by the familiar experiment of quickly inserting the phosphorus 

 end of a match into the center of the flame, where it may sometimes 

 be held until the wood of the match is burned through without taking 

 fire. The flame can not spread inward, because there is nothing there 

 to support combustion ; nor outward, because there is nothing there to 

 burn. The flame is simply that part of the current of gases where the 

 chemical action takes place, and where, consequently, the heat is pro- 

 duced. It is more nearly a place than a thing. If we leave out of 

 account for a moment the chemical changes, we may compare the cur- 

 rent of gas which flows from the tube to a metallic rod which is being 

 slowly pushed through a fire. The portion in the fire glows, and, as 

 the rod moves on, different portions of it glow, while the glowing spot, 

 which may be compared to the flame, remains stationary. In the hy- 

 drogen-flame the rod is of gas and invisible. We see only the spot 

 which glows, and, as this is stationary, we are apt to regard it as an 

 object by itself, instead of considering it as a spot in a constantly flow- 

 ing stream. 



The hydrogen-flame gives out very little light. If burned from a 

 metallic jet, the flame becomes almost invisible. What little light 

 such a flame does emit, it emits because the gases of which it is com- 

 posed are hot ; but ordinary gases, when heated to the temperature of 

 a flame, emit very little light. 



To render a flame luminous in the sense in which an ordinary gas- 

 flame is luminous, we must introduce into it some solid which is not 

 converted into gas at the temperature of the flame. The light of a 

 common gas-flame is due to innumerable small particles of carbon, 

 which are separated from the gas in the interior of the flame and are 

 heated white hut while passing through the flame. Illuminating gas 



