Wet wood or wet coal we know will not burn, or can only be made to do so with difficulty. Other kinds of burning or oxidation are, however, well known, which take place in the presence of moisture. The rusting of iron, for example, is an oxidation or burning of the metal, and takes place faster in damp air than in dry; during the slow rusting in moisture just as much heat is set free as if the same com-pound of iron and oxygen were prepared in a more rapid way. Such experiments throw great light on the oxidations which take place in our own bodies. All of them are slow oxidations, which never at any one moment give off a great amount of heat, and all occur in the damp tissues.

How does the rate of oxidation differ in the two cases ? How does the oxidation of arrowroot burned in a fire differ from its oxidation in the living body?

Can oxidations occur in the presence of moisture ?

Summary. (1) Energy can be turned from one form into another; as from heat into mechanical work by a steam-engine. (2) Our bodies are constantly losing energy, partly in muscular work, and partly as heat lost to surrounding objects. (3) Energy cannot be created; all that can be done is to turn one kind of it into another kind: heat can be turned into mechanical work (as in a locomotive) ; or mechanical work into heat (as by friction) ; or heat into electricity (as in a thermo-electric machine); and so forth. (4) Since our bodies spend energy all our life long they must be supplied with it from outside : they can turn into other forms the energy which they receive, but they cannot make it from nothing. (5) The chief forms of energy which our bodies expend are muscular (i. e. mechanical) work, and heat. (6) In ordinary machines, as a locomotive, the source of the work done and the heat given off is the oxidation of coal in a furnace. (7) Chemistry teaches us that just the same amount of energy or work power is given off when an ounce of any given substance is oxidized, whether the oxidation occurs rapidly or slowly. (8) Chemistry also teaches us that many oxidations, or burnings, occur in the presence of water, and that in them just the same amount of energy is set free as if the oxidation occurred in dry air. (9) In our bodies substances are burnt slowly at a low temperature and in the presence of moisture: in this burning energy is set free which the body uses for performing its necessary work. (10) In the burning which the tissues undergo while they work they are used up and destroyed. (11) To compensate for the destruction of tissue which accompanies and provides the power for every action of the body, material must be taken into it from outside, which will restore or repair the oxidized tissues. (12) Such substances taken into the body from outside are called foods, and the constant oxidation of the body which is necessitated by the performance of the functions essential to life, requires a supply of food from the outer world, if life is to be maintained, (13) The body of a healthy person has in it at any moment a certain reserve of oxid-izable matters, which we may call stored-up food. The most important of these reserve foods is fat. A fat map. can accordingly bear starvation longer than a lean one under similar circumstances.

Give an instance. Does the rate of oxidation or the presence of moisture affect the amount of heat liberated ?

Of what kind are the oxidations which occur in our bodies ?

Give a summary of the contents of this chapter with reference to the following points: (1) The transformation of energy; (2) The loss of energy from the body ; (3) The fact that man cannot create energy but can transmute it; (4) The fact that our bodies must be constantly supplied with energy from outside ; (5) The chief forms of energy spent by the body ; (6) The source of the energy spent by a working steam-engine.

(7) The amount of energy given off when a substance is oxidized at high or low temperatures; (8) The teachings of chemistry with reference to oxidations in the presence of moisture ; (9) The conditions under which substances are oxidized in our bodies ; (10) The changes which oxidized tissues undergo ; (11) Why we need to take material from the outer world into our bodies ; (12) What is meant by food; and why we need foods ; (13) What are reserve foods. Illustrate.