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"O F all the blessings we enjoy, the first place should be given to health, which is constantly menaced by divers perils, and these are all the more to be feared when they are unknown. Let us learn what these dangers are and the means of avoiding them; then we shall be able to preserve our health, so far as its preservation depends upon ourselves.
"Prominent among the needs to which we are subject stand the need of food, the need of drink, and the need of sleep. But there is still another before which hunger and thirst, however violent they may be, lose their importance; a need continually born anew and never satisfied; a need that knows no respite and makes itself felt whether we wake or sleep; by day, by night, and all the time. It is the need of air.
"So necessary is air to the maintenance of life that it has not been left to us to control our use of it as we do in the case of food and drink. Unconsciously, and with no volition on our part, we admit the air to our lungs and allow it to play its wonderful part in our system. On air, more than on anything else, we live, our daily bread taking only second place. The need of food is felt only at comparatively long intervals; the need of air is felt uninterruptedly, ever imperious, ever inexorable.
"To convince yourself of this, try for a moment to suspend the admission of air to your lungs by closing the doors against it, the nose and the mouth. In a few seconds you will be forced to end the experiment; you will begin to stifle and will feel that death would surely follow if you persisted in your experiment.
"All animals, from the smallest to the largest, are in like case with ourselves: first and foremost they live on air. Not even do those that live in water—the fishes and other forms of aquatic life—make any exception to the rule: they cannot live except in water containing a certain amount of air.
"There is a striking experiment in physics to illustrate this point. Some small living creature—a bird, for example—is put under a bell-glass from which the air is being gradually exhausted by means of an air-pump. As the supply of air diminishes under the action of the pump, the bird begins to totter, struggles in an anguish painful to see, and finally falls in the death agony. Unless air is quickly admitted once more to the bell-glass, the poor victim will be dead and nothing can restore it to life. But if air is admitted in time, it re-animates the bird. Again, if a lighted taper instead of a live bird be placed under the bell-glass, the flame is extinguished as soon as the air is withdrawn. The bird must have air if it is to live; the taper if it is to burn.
"What I am now going to tell you will explain briefly the reason for this necessity for air. Man and animals have a temperature suitable to them, a degree of warmth resulting, not from any outer circumstances, but from the vital processes within. Clothing helps to retain this warmth, helps to prevent its dissipation, but does not supply it. Moreover, this natural warmth is the same under a burning sun and amid the frosts of winter, in the hottest of climates and in the coldest. Finally, it cannot be lessened without placing us in very serious danger. In the case of man its measurement on the centigrade thermometer is thirty-eight degrees.
"How is it that this warmth of the body is kept always and everywhere the same; and whence can it come if not from combustion? As a matter of fact there is going on in us a continual combustion, supplied with fuel in the form of food by our eating, and furnished with the necessary oxygen from the air we breathe. To live is to be burnt up in the strictest sense of the word; and to breathe is to burn. From time immemorial there has been in use, in a figurative sense, the expression, 'the torch of life.' We now perceive that this figurative expression is in reality the literal expression of the truth. Air makes the torch burn, and it also makes the animal burn. It causes the torch to give out heat and light, and it causes the animals to produce heat and motion. Without air the torch becomes extinct; without air the animal dies. In this respect the animal may be likened to a highly perfected machine set in motion by the heat from a furnace. The animal eats and breathes in order to generate heat and motion, receives its fuel in the form of food, and burns it up in its body with the aid of air supplied by breathing.
"We say that animals eat and breathe to generate heat and motion. That is why the need of food is greater in winter than in summer. The body cools off more rapidly in contact with the cold air outside, thus making it necessary to burn more fuel in order to keep up the natural warmth. A cold temperature, therefore, whets the appetite for food, while a warm one tends to dull its edge. The famishing stomachs of dwellers in the far North demand hearty food, such as fat meat and bacon; but the tribes of Sahara are satisfied with a daily ration of a few dates and a small portion of flour kneaded in the palm of the hand. Anything that lessens the loss of heat lessens also the need of food. Sleep, rest, warm clothing, all these serve to supplement the taking of nourishment and to conserve the natural heat of the body, even in a certain sense taking the place of nourishment. This truth finds expression in the common saying that he who sleeps dines.
"The fuel burnt up in us by the air we breathe is furnished by the very substance of our bodies, or more particularly by the blood, into which the food we digest is transformed. Of a person who applies himself to his work with excessive ardor we say that he burns the candle at both ends—another popular phrase that could not be bettered in its agreement with what is most assuredly known concerning the vital processes. Not a movement is made by us, not a finger is lifted, without causing a consumption of fuel proportioned to the energy expended; and this fuel is furnished by the blood, which itself is maintained by the food we eat. Walking, running, working, putting forth effort, engaging in activity of any sort—all these do in a very real sense burn the bodily fuel just as a locomotive burns its coal in hauling after it the heavy burden of its long train of cars. Thus it is that exercise and hard work increase the need of food, whereas rest and idleness diminish this need.
"The coal in a furnace takes fire, becomes red-hot, and burns up, at the same time giving out heat. Soon there is nothing left of it but a quantity of ashes weighing much less than the coal consumed. What has become of the part represented by this difference in weight? I have already told you in my talk on combustion. It is no longer in the furnace in black lumps visible to the eye, but it is in the air in a form that the eye cannot see.
"Air, as chemistry tells us, is a mixture of two gases having very different properties the one from the other. These gases are oxygen, an active gas lending itself readily to combustion, and nitrogen, an inert gas with no tendency to combustion. In one hundred liters of this atmospheric mixture there are twenty-one liters of oxygen to seventy-nine of nitrogen. Now, in burning, the carbon of coal unites with the oxygen of the air, the two forming a gaseous compound called carbonic acid gas, which becomes diffused in the atmosphere. The part of the coal that is not carbon remains in the furnace, being insoluble in the atmosphere, and constitutes the ashes. All the carbon, then, disappears, seeming to undergo annihilation because we no longer see it, just as we cease to see the lump of sugar dissolved in water. This dissolution in oxygen, with the generation of heat, is called combustion.
"The incessant combustion going on in our bodies at the expense of the materials furnished by the blood is by no means comparable for violence with the combustion taking place in a furnace. It is a slow burning, somewhat like the spontaneous ignition of a damp hay-mow before it bursts into flame. It produces heat, but not enough to endanger the body as it would be endangered by undue proximity to a glowing furnace.
"In passing through a furnace and maintaining the fire therein, air changes its nature: its oxygen unites with carbon to form carbonic acid gas, which escapes through the chimney, while pure air is continually entering to take its place. Exactly the same process goes on in the combustion that keeps us alive. The lungs act as a pair of bellows, alternately filling themselves with air and emptying themselves. These alternating movements are known as inspiration and expiration. In the first, pure air is drawn in to burn up certain constituents of the blood and generate heat; in the second the air, after performing its office, is expelled, not the same in substance as when it entered, but impregnated with carbon and unfit for breathing, like the air escaping through the chimney from a furnace. The nitrogen in the air undergoes no change, but carbonic acid gas takes the place of most of the oxygen. In short, the breath from our lungs is essentially the same as the breath from a furnace."