"Of the three things that are indispensable for the subsistence of man, — namely, air, water, and food — the first is the most important. We should all be as much against the breathing of impure air as we are against the drinking of dirty water and the eating of dirty food; but the air we breathe is, as a rule, far more impure than the water we drink or the food we eat. ... In fact, every child should be taught the value of fresh air, as soon as it is able to understand anything. ... All men should learn to breathe through the nose alone. And this is not at all difficult, if we remember to keep our mouth firmly shut at all times, except when we are talking. After learning how to inhale the air, we should cultivate the habit of breathing fresh air, day in and day out. (Chapter III, Air)"