THE VOLATILE PART OF PLANTS. 19 



we employ its behavior toward other bodies for its recog- 

 nition. 



EXP. 5. Place a burning splinter of wood in a vessel of oxygen (lifted 

 for that purpose, mouth upward, from the water). The flame is at once 

 greatly increased in brilliancy. Now remove the splinter from the 

 bottle, blow out the flame, and thrust the still glowing point into the 

 oxygen. It is instantly relighted. The experiment may be repeated 

 many times. This is the usual test for oxygen gas. 



Combustion. When the chemical union of two bodies 

 takes place with such energy as to produce visible phe- 

 nomena of fire or flame, the process is called combustion. 

 Bodies that burn are combustibles, and the gas in which 

 a substance burns is called a supporter of combustion. 



Oxygen is the grand supporter of combustion, and 

 nearly all cases of burning met with in ordinary experi- 

 ence are instances of chemical union between the oxygen 

 of the atmosphere and some other body or bodies. 



The rapidity or intensity of combustion depends upon 

 the quantities of oxygen and of the combustible that 

 unite within a given time. Forcing a stream of air into 

 a fire increases the supply of oxygen and excites a more 

 vigorous combustion, whether it be done by a bellows or 

 result from ordinary draught. 



Oxygen exists in our atmosphere to the extent of about 

 one-fifth of the bulk of the latter. When a burning body 

 is brought into unmixed oxygen, its combustion is, of 

 course, more rapid than in ordinary air, four-fifths of 

 which is a gas, presently to be noticed, that is compara- 

 tively indifferent in its chemical affinities toward most 

 bodies. 



In the air a piece of burning charcoal soon goes out ; 

 but if plunged into oxygen, it burns with great rapidity 

 and brilliancy. 



EXP. 6. Attach a slender bit of charcoal to one end of a sharpened 

 wire that is passed tlu-ou^h a wide cork or card; heat the charcoal to 

 redness in the flame of a lamp, and then insert it into a bottle of oxy- 

 gen, Fig. 3. When the combustion lias declined, a suitable test applied 



