Who can fully estimate the value of health? It affects not only the happiness, but also the usefulness of every life. Without it, no substantial success can be achieved. By due attention to the simple laws of health, involving fresh air, pure water, wholesome food, sensible clothing, proper exercise, rest, and sleep, nine-tenths of all the ailments that afflict mankind, and the largest amount of human misery resulting therefrom, would be prevented. AIR =Essentials of Life.=--Air, water, and food are the great essentials of life. A man may go for days without food, and for hours without water, but deprive him of air, even for a few minutes, and he ceases to live. In quantity, the daily consumption of air far outmeasures the other two; in purity, it receives the least consideration. The city and the State alike exercise some oversight of the food and water supply of the people. Impurities in these often appeal to the sense of sight or smell or taste, and the individual is put on his guard. The intangible air is laden with the foulest and most poisonous substances, and is as freely inhaled as if it could make no difference to the health. =Lung Capacity.=--The quantity of the air we breathe is also important. We may eat too much food, even though it be absolutely pure and wholesome, but we cannot consume too much pure air. The larger the lung space, therefore, the better for health and strength. The full lung capacity of the average adult is about 330 cubic inches, but an ordinary inspiration does not take in more than one-eleventh part of that volume. The value of full, deep breathing, and of large lung capacity becomes at once apparent. The larger the quantity of air consumed, the greater the amount of life-giving oxygen conveyed through the blood to all parts of the body. No form of physical exercise, therefore, can exceed in value the breathing exercises described in another chapter. =Rate of Breathing.=--It is estimated that we breathe once during every four beats of the heart, or about eighteen times a minute. The relation between the heart and lungs is so close that whatever modifies the pulse affects the breathing. When the heart action is hurried, more blood is sent to the lungs, requiring more rapid action on their part. About every fifth breath the inspiration is longer and fuller, the effect being to change more completely the air of the lungs. =Holding the Breath.=--While respiration is, for the most part, involuntary, we may arrest the breathing for the space of twenty to thirty seconds. If we first fortify the lungs by taking several deep inspirations and expelling the impure air as fully as possible, we may hold the breath for a minute or two. This power will prove of advantage if we have occasion to pass through a room or hallway filled with smoke, or to remain under water for a brief time. The pearl-fishers, as a result of training, remain under water from three to four minutes.