COMPOSITION OF AIR ATOMIC THEORY 71 



The substance which we call carbon is so familiar to us 

 as electric-light carbons, charcoal, lampblack, soot, etc. that 

 it is not necessary to describe it. If a cold object is held in 

 the flame of a candle for a moment, carbon (soot) is de- 

 posited upon it. 



Some of the carbon may be seen to pass upward from the 

 tip of the flame as smoke, but if the candle is well trimmed, 

 there should be little or no smoke. Carbon as well as hydro- 

 gen is able to unite with oxygen, and though there is much 

 carbon in the flame, very little of it passes away from the 

 flame in the form of soot. It unites with the oxygen of the 

 air to form a compound made up of carbon and oxygen 

 known as carbon dioxide. 



74. Carbon dioxide formed by the flame. If a candle or 

 other flame is allowed to burn for a time in a bottle of air, 

 and if limewater is then poured into the bottle and shaken 

 slightly, the limewater will become milky. If we do the same 

 with a bottle of air in which the flame has not burned, the 

 limewater remains clear. This shows that there is something 

 in the bottle that was not there before the flame was put 

 into it Chemists tell us that the substance which caused 

 the limewater to become milky is a gas composed of carbon 

 and oxygen. Is it an element or a compound ? We may 

 burn some charcoal (carbon) in a jar filled with pure oxy- 

 gen. In that case anything that is formed can contain only 

 carbon and oxygen, since there is nothing else in the jar. 

 If we test the results of this burning, as before, the lime- 

 water turns white, showing us the evidence of the same 

 substance. This substance which is produced by the burning 

 candle is a compound of carbon and oxygen and is formed 

 by the union of the oxygen of the air and the white-hot 

 carbon particles in the flame. 



The proper abbreviated symbol or formula by which to 

 represent a molecule of this carbon-dioxide gas is CO 2 . What 

 does this formula mean? 



