THE AIR OF TOWNS. 359 



Lecture 2. — Smoke. 



Smoke is solid matter given off during burning. Gunpowder smoke 

 is largely mineral salts and so is tobacco smoke. Coal smoke is soot — 

 that is mainly what chemists call carbon. All the common inflammable 

 substances, coal, wood, paraffin, petroleum, benzine, as well as coal gas, 

 contain carbon and in luminous flames the carbon can readily be shown 

 as soot. I have only to bring this white plate into the candle flame 

 and Ave have as you see at once a deposit of soot. This soot in the 

 flame is white hot and gives to the flame its luminosity. The lumi- 

 nosity imparted by solid matter to a nonluminous flame may be readily 

 demonstrated. 



Here is a blow pipe (fig. 13), fed with coal gas and oxygen, which 

 gives as you see a nonluminous flame like burning spirits of wine, but 

 it is nevertheless a very hot one, for as soon as I introduce a lump of 

 infusible material, like quicklime, the latter becomes in a moment 

 white hot and brilliantly luminous. 



But an ordinary luminous flame is not necessarily a smoky one, 

 because the soot burns when it reaches the outside of the flame and 

 comes into contact with the air. 



Why is it, then, that luminous flames are sometimes smoky and some- 

 times not? Coal and wood, benzine, paraffin, turpentine, and often 

 tallow and wax candles burn and give off soot. It is because there is 

 too little air where the flame is hottest. The soot as it passes up gets 

 cool and when it reaches a new air supply it is too cold to take fire. 

 It is this that makes a candle, with a wick that requires snuffing, give 

 a smoky flame, because with the long wick it is supplying more 

 combustible to the flame than the surrounding air can burn. 



An ordinary oil lamp smokes until the chimney is put on. Then the 

 draft up the chimney is increased, more air is supplied, the flame gets 

 hotter and therefore brighter, and the soot is burned up. 



Here is a smoky turpentine flame. By blowing oxygen through the 

 center a brilliant nonsmoky flame is produced. 



In a, fig. 14, we have a section of the apparatus. It consists of a 

 metal tube, furnished at the top with a hollow metal rim, which is filled 

 with cotton wool soaked in turpentine; h represents the smoky tur- 

 pentine flame and c the flame after admission of oxygen. 



Soot or coal smoke is then an inflammable part of the fuel and where 

 soot is allowed to escape, the fuel is lost. If, then, we not only feed the 

 flame with more air, but at the same time make the soot hot the smoke 

 is consumed. These are the two simple principles of smoke preventfon. 

 Let me show you this by an experiment with a model furnace, flue, and 

 chimney (fig. 15). This consists of a straight metal pipe open at both 

 ends and perforated with air holes near the lower end. A bent metal 

 arm is fixed on by a T piece and represents the flue. The furnace is 

 represented by a turpentine lamp, which burns inside the sheet-iron 



