382 THE POPULAR SCIENCE MONTHLY. 



of heat, and would therefore not convey away the heat of the flame so 

 rapidly as metal. Experiment shows, however, that the difference is 

 very slight. It is measurable, but not of practical importance. The 

 practical value of a lava-tip lies simply in the fact that it does not 

 rust. Neither are any arrangements for heating the burner likely to 

 effect any gain in illuminating power. Obviously the heat for this 

 purpose must not be taken from the flame itself, but must either be 

 obtained from its waste heat or from a separate source of heat. The 

 waste heat of the flame would probably not heat the burner hot 

 enough to produce any appreciable effect, while a separate source of 

 heat involves additional expense enough to overbalance any gain likely 

 to accrue. 



A flame may be cooled in another way than by means of some cold 

 body held in it, namely, by mixing some indifferent gas with the 

 combustible gas. If, instead of air, a stream of carbonic-acid gas be 

 admitted at the bottom of a Bunsen lamp, the flame becomes " non- 

 luminous." We get the same amount of heat in the flame as before, 

 since we burn the same amount of gas ; but this heat has not only to 

 be applied to heating the illuminating gas and the products of its 

 combustion, but has also to heat the carbonic-acid gas. As a conse- 

 quence, no part of the flame can be so hot as it was before, and the 

 separation of carbon does not take place. 



If the tube through which the gases pass be heated, the heat thus 

 added counteracts the cooling effect of the carbonic-acid gas, and the 

 flame becomes luminous again. 



2. Dilution. The second way in which we may render a gas-flame 

 " non-luminous " is by dilution, either of the gas or the air. This can 

 be shown by substituting for the carbonic-acid gas some combustible 

 gas which does not give a luminous flame ; for instance, carbonic-oxide 

 gas. This gas can not essentially lower the temperature of the flame, 

 for it burns with as hot a flame as gas, yet the mixture of it with illu- 

 minating gas gives a " non-luminous " flame. Plainly, then, we must 

 conclude that simple dilution is able to produce this effect. Appar- 

 ently such a mixture as this that is, a mixture of two gases, one of 

 which burns with a luminous and the other with a "non-luminous" 

 flame requires to be made hotter in order to produce a separation of 

 carbon, and consequently a luminous flame, than does the illuminating 

 gas when burned alone. If we raise the temperature of the flame by 

 heating the burner, it becomes luminous again. Dilution, then, renders 

 a flame "non-luminous," because the temperature of the flame is not 

 high enough to cause a separation of carbon from the diluted gas. 



3. Too Rapid Oxidation of the Carbon. The third method of 

 rendering a flame non-luminous is to supply it with so much oxygen 

 that the carbon is burned at once before it can separate in the solid 

 form. 



If we put a small gas-flame into a jar of pure oxygen it is rendered 



