COMBUSTION. 



345 



blue. We have still something to learn about ozone, which may be considered 

 as "condensed oxygen." 



We have frequently mentioned "combustion," and as under ordinary 

 circumstances such effects cannot take place without atmospheric air, we will 

 consider it. Combustion is chemical action accompanied by light and heat 



Development of gas by combustion. 



Fig- 327- 



Chemical union is always attended by the development of heat, not always 

 by light, because the union varies in intensity and quickness. But when a 

 candle is burning we can study all the interesting phenomena of combustion. 

 We have already spoken of HEAT 

 and LIGHT, so we need only refer 

 the readers to those subjects in the 

 former parts of this volume. Heat 

 is referable to chemical action, and 

 varies according to the energy of 

 union. Heat is always present, re- 

 member, in a greater or less degree ; 

 and when visible combustion takes 

 place we see light. Invisible com- 

 bustion goes on in our bodies, and 

 we feel heat ; when we get cold we 

 feed the fire by eating, or blow it 

 by exercise and air in our lungs. 



We shall speak, however, of 

 combustion now as it affects us in daily life ; our fires, our candles, gas, etc., 

 and under these ordinary circumstances hydrogen and carbon are present. 

 (We shall hear more about carbon presently.) These unite with the oxygen 

 to form water and carbonic acid ; the water being visible as we first put the 

 cold shade upon the lighted lamp, and the carbonic acid renders the air impure. 



Fig. 328. Gas evolved from flame. 



