You burn a piece of wood or coal to get heat; but what makes your body hotter than the air outside? Why, when the doctor thrusts a thermometer into you, does the instrument register a temperature of 98 - and sometimes a few degrees higher, if you have the "flu" for example - though the temperature of the room is much lower? Is your inside an imitation of a fireplace? Even if it is, the source of heat is certainly neither coal, nor wood, nor paper, nor anything else that is commonly used as fuel.

Such questions have agitated the minds of thinking men from the remotest times, but only within the last century or so have satisfactory answers been found. The guess that the body had certain analogies to a furnace was a good one; but before we could solve the riddle of the body furnace, we had to acquire clearer notions of just what this "burning" is that takes place in the ordinary fireplace.

If you want to make a fire you of course have to have a fuel. But equally important is the presence of enough air. If your clothes by any accident catch fire, you are warned to throw a wrap tightly around you, so as to prevent access of air. Without air there can be no burning, no matter how much coal or wood there may be.

Oxygen

But what is there in the air that is so essential to burning? The chemist tells us that it is the oxygen. This gas is present in the air to the extent of about twenty per cent. If a sample of air be taken and the oxygen removed from it, your paper will not burn; nor will anything else that ordinarily burns in the air. If, on the other hand, you take your burning paper or lighted candle, and thrust it into a jar containing the removed oxygen, the paper or candle will burn with a brilliancy that dazzles the eye.

Priestley, an Englishman, who later took refuge in Pennsylvania to escape from religious persecution, first isolated this wonderful oxygen in 1771, but it remained for Lavoisier, a Frenchman, to show just in what way this gas is related to the process of burning, and to the process of respiration or "burning in the body." He did this work while the French Revolution was doing its work; and he was rewarded for his labors by being guillotined.

Lavoisier

Lavoisier showed that if you take a piece of coal and burn it, the carbon and the hydrogen, the two chief elements in the coal, combine with oxygen, forming carbon dioxide and water respectively; thus carbon plus oxygen yields carbon dioxide; and hydrogen plus oxygen yields hydrogen oxide (commonly known as water); and that as a result of this combination, a large amount of heat is evolved.

Lavoisier next showed that much the same thing takes place when food is taken into the body. Here also the carbon and hydrogen in food - just as certainly present in meat and bread as in wood and coal - combine with the oxygen in the air obtained by breathing, to yield carbon dioxide and water, at the same time liberating heat.

That we actually liberate carbon dioxide and water can be easily shown. Take a straw used for drinking a soda and blow through it into a glass containing lime water; the lime water will immediately turn milky. The same is true if you thrust a lighted candle into a jar, keep it there for a few seconds, then take it out and add a little lime water to the jar and shake. In either case the chemist can prove to you that it is the carbon dioxide released from your body or from the lighted candle that turns the lime water milky.

Likewise if you blow on a cold surface, say your glasses, the surface becomes moist. If you burn your candle surrounded by a tall glass chimney, you will notice that the upper portion of the chimney becomes moist; this moisture, to be sure, soon disappears, but that is due to the heat from the candle.

Just as heat is produced when the carbon and hydrogen from the candle or coal unite with the oxygen to form carbon dioxide and water, so heat is produced when these elements in the food we eat unite with the oxygen in the air we breathe to produce the same products.

Now we know why the doctor's thermometer thrust in your mouth registers a higher temperature than the same thermometer hung in the room. And just as the coal gives the heat and therefore the energy necessary to convert the water in the boiler into steam and so run the engine, so probably the food we take into our system gives us the energy needed to carry on our daily work.

Measuring Heat

Obviously enough, the value of the fuel must depend primarily upon the amount of heat you can get out of it. If one ton of coal mined in Pennsylvania gives you one and one-half times as much heat as a ton of coal mined in Wales, - if there are more carbon and hydrogen and less impurities in one sample than in another - then you will turn to Pennsylvania for your coal supply; provided, of course, the Welsh coal is not so much cheaper as to offset the increased fuel value of the Pennsylvania coal.

The question, then, of how much heat you can get out of a ton of coal - or, as the coal merchants and chemists put it, what is the fuel value of one ton of coal - becomes of paramount importance.

And if the value of the coal lies in the amount of heat you can get out of it, may not the value of food depend upon the amount of heat it produces when "oxidized" in the body? Hence the importance attached to a means for measuring heat.