53 SCIENCE IN SHORT CHAPTERS. 



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, 

 whether completely or incompletely. If the air supplied for 

 its combustion is insufficient, 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 depends 

 upon how we 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 combustion 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 order to obtain smokeless 

 fires is to secure the complete combustion 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 is 

 the combustion, or burning. 



Therefore, when the coal gas and the air are brought 

 together, one or the other, or both, must be heated up to a 

 certain point in order that the combustion be complete. If 

 cold, there is no combustion ; if insufficiently heated, there is 

 imperfect combustion, however well the supplies may be 

 regulated. 



A very simple experiment that anybody may make illustrates 

 this. When an ordinary open fire is burning brightly and 

 clearly without flame, throw a few small pieces of raw coa 

 into the midst of the glowing coals. They will flame fiercely, 

 but without smoking. Then throw a heap of coal or one large 

 lump on a similar fire. Now you will have dense volumes of 

 smoke, and little or no flame, simply because the cooling 

 action of the large bulk of coal in the course of distillation 

 brings the temperature of its gases below that required for 

 their complete combustion. 



This simple experiment supplies a most important p; apical 



