GAS WHICH PLANTS GIVE OFF 123 
is necessary in making a fire so that it will consume 
the wood, or coal, or other material. 
The reason the glowing splinter does not flame in 
the air is because the proportion of oxygen is not great 
enough to ignite it. 
But there is so much 
oxygen caught in the 
test tube from the plant 

that the glowing coal Fic. 155. The at inten lights again in the 
readily flames again. of ie See 
How is the gas formed? It 1s difficult to show here 
just how this gas is formed, for a considerable knowledge 
of chemistry is necessary to understand it thoroughly. 
But perhaps you have learned about some of the chem- 
ical compounds, as they are called, and how they some- 
times change their combinations and associations. First 
let us boil some water. When it 1s cool, put a water 
plant in it and set it in the sunlight. No gas-is given 
off. This is queer behaviour, you may say. But it 
shows us that something was In the water which the 
boiulmg drove off, and which is necessary for the plant 
in order that the oxygen may be set free. This was 
air and carbonic acid.! If we introduce air and carbon 
dioxid into the water, oxygen will soon be given off again 
by the plant, since it can now absorb carbonic acid. 
1 The carbon dioxid is here in the form of carbonic acid, since it is 
in water, 
