COMBUSTION". 141 



a fire, we bring more air, and therefore more oxygen, to it. 

 The coals therefore brighten, the combustion being made 

 more active, and this increasing the heat, the wood burns 

 more briskly, or, if not burning at all, soon bursts into a 

 flame. It is chiefly for the same reason that when a build- 

 ing takes fire there is great danger that the fire will extend 

 to other buildings. So, also, whatever increases the draught 

 of a chimney makes the fire more brisk. On this account 

 the chimneys of foundries and other factories, in which 

 a very hot fire is needed, are made very tall. For the same 

 reason the tall chimneys of lamps cause them to give a very 

 bright light. If you should take the chimney from a lamp 

 that is burning brightly, leaving the wick at the same height, 

 there would be a great smoke, because the oxygen would 

 not come to the wick with sufficient rapidity to unite with 

 all the carbon and hydrogen that go up from it. A flat 

 wick gives a brighter light than a round one, because it 

 presents a larger surface to the oxygen of the air. Still 

 more light is given if a flat wick have a circular arrange- 

 ment, the air being admitted inside as well as outside of 

 the circle. This is the construction of the well-known Ar- 

 gand burner. 



181. Bunsen's Burner. By increasing the supply of oxy- 

 gen to a flame we increase its luminosity ; but if we mix 

 the combustible gases with oxygen before igniting them, 

 the resulting flame gives scarcely any light at all. This 

 will not seem so strange if you understand that the par- 

 ticles of carbon are completely burned np in the mixed 

 gases. Such a flame is not only smokeless, but deposits no 

 soot on cold surfaces placed in it, and consequently is a very 

 clean flame to cook or heat with. Many forms of stoves 

 and lamps have been contrived which produce this color- 

 less and very hot flame ; the one commonly used in chem- 

 ical laboratories is called Bunsen's Burner, after the great 



