SCIENCE OF COMMON THINGS. 223 



W hen will a lamp smoke ? Benefit of glass chimney on lamp. 



There are / and if we present a cold surface to the 

 flame, they become deposited on it in the form of soot. 



1484 When we Say a lamp smokes, what do we mean * 



That the solid parts of the flame are passing off in an 

 unconsumed state, 



1485 When the flame burns properly, why does the smoke cease to be 

 emitted f 



Because the solid particles of carbon constituting the 

 smoke are burned up, or are completely united with 

 oxygen, forming an invisible gas carbonic acid. 



1480 From what source is the carbon, constituting the illuminating par- 

 ticles in tiie one case and the soot in the other, derived? 



It was originally a part of the burning or combustible 

 body, 



148 7 When will a flame smoke f 



When the supply of oxygen received from the air ig 

 insufficient to consume all the carbon which the heat 

 separates from the combustible body in the form of 

 soot. 



1488 What benefit arises from surrounding aflame with a glass cylin- 

 der of chimney open at the bottom and top ? 



When a flame burns without a chimney, the hot air 

 radiates in all directions ; but when it is surrounded 

 by a chimney, the hot air is confined within the walls 

 of the cylinder: consequently, the hot air will issue 

 rapidly from the top of the chimney, and cold air will- 

 enter equally fast at the bottom to replace it. In this 

 Way a constant current of fresh air is kept up through 

 the centre of the flame, causing a more perfect com- 

 bustion, and a brighter and stronger flame. 



148Q Why in solar and astral lamps do we use a hollow or circular 

 wick? 



In order that a current of air may rusn up through the 

 interior surface of the flame as well as aiong the exterior. 



1490 What is fuel? 



Any substance which serves as aliment or food for 

 '(ire. In ordinary language we mean by fuel the pecu- 

 liar substance of plants, or the products resulting from 



