28 BOTANY 
the coal very rapidly and much heat and lhght are 
produced. On the other hand, when oxygen combines 
with iron to form rust, we have slow oxidation, and, 
though the total heat given off is considerable, it is 
produced so slowly that the iron never gets appreciably 
hotter. Nevertheless, if a coil of fine wire be heated 
red-hot at one end and held in a jar of oxygen, it will 
burn fiercely and produce much heat in a short time. 
The great function of oxygen in the living world is 
to oxidise things and set free heat and other forms of 
energy. Energy is the power to do work. All know 
the work that can be done by the burning or oxidation 
of coal. The heat given off may be used to warm a 
room, drive a locomotive, or to produce the electricity 
necessary for lighting or for driving a tram ear. 
Neither animals nor plants could exist without oxygen. 
Animals absorb it through their lungs and use it to 
oxidise the food they have taken through the mouth, 
and thus to set free the energy they need for the body 
work. Plants, as we shall see later, use oxygen for 
the same purpose. 
Carbon is a solid substance found in every tissue 
that goes to make up plant*or animal. The diamond 
is pure carbon, while graphite, charcoal, coal, and 
coke are more or less impure forms of the same 
substance. The presence of carbon in any organic 
material may be indicated by heating it in a test tube, 
when the presence of carbon is shown by the charring 
of the substance. Thus milk, meat, wood, leaves, sugar, 
starch, and bone will all char when heated, for all 
contain carbon. In most plants, at least one-half the 
dry weight consists of carbon. 
_ Carbon-dioxide is a gas which is usually prepared 
in the laboratory by pouring hydrochlorie acid on 
marble chips. If these materials are not available the 
student may, however, obtain the gas by pouring 
