328 SCIENCE IN SHORT CHAPTERS. 



feet combustion, and, knowing this, it can easily be under- 

 stood that to return the carbonic acid and excess of carbon 

 to the already suffocated fire can only add smother to smoth- 

 eratioii, and"make the smoky fire more smoky still. 



There is, however, one case in which a lire appears to 

 thus consume its own smoke, but the appearance is delu- 

 sive. I refer to fires lighted from above. These, if pro- 

 perly managed, are practically smokeless, and it is com- 

 monly supposed that smoke passes from the raw coal below 

 through the burning coal above, and is thereby consumed. 

 The fact is, however, that no such smoke is formed. That 

 which under these conditions comes from the coal beneath, 

 when gradually heated by the fire above, is combustible gas, 

 and this gas is burned as it passes through the fire. In 

 this case the formation or non-formation of smoke depends 

 mainly on how this gas is burned, whethey completely or 

 incompletely. If the air supplied for its combustion is in- 

 sufficient, smoke will be formed as it is when we turn up 

 an Argand gas-flame so high that the gas is too great in 

 proportion to the quantity of air that can enter the glass 

 chimney. 



Herein lies the fundamental principle. We may prevent 

 smoke, though we cannot cure it, and this prevention de- 

 pends upon how \ve supply air to the gas which the coal 

 gives off when heated, and upon the condition of this gas 

 when we bring it in contact with the air by which its com- 

 bustion is to be effected. We must always remember that 

 coal when its temperature is sufficiently heated, whether in 

 a gas retort or fireplace, gives off a series of combustible 

 hydrocarbon gases and vapors, and all we have to do in or- 

 der to obtain smokeless fires is to secure the complete com- 

 bustion of these. 



Now we know that to burn a given quantity of gas we 

 must supply it with a sufficient quantity of oxygen, i.e., of 

 the active principle of the air; but this is not all: we all 

 know well enough that if cold coal-gas and cold air be 

 brought together in any proportion whatever no combustion 

 occurs. A certain amount of heat is necessary to start the 

 chemical combination of oxygen with hydrogen and carbon, 

 which combination i.s the combustion, or burning. 



