THE VOLATILE PART OF PLANTS. 35 
As this gas makes no peculiar impressions on the senses, 
we employ its behavior towards other bodies for its recog- 
nition. 
Exp. 5.—Place a burning splinter of wood in a vessel of oxygen (lift- 
ed 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 all 
the cases of burning met with in ordinary experience are 
instances of chemical union between the oxygen of the at- 
mosphere and some other body or bodies. 
The rapidity or intensity of combustion depends upon 
the quantity 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 vigor- 
ous combustion, whether it be done by a bellows or re- 
sult 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 nearly in- 
different 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 through a wide cork or card; heat the charcoal to 
redness in the flame of a lamp, and then insert it into a bottle of oxygen, 
