Among all the millions of outside agencies that go to build up and strengthen, or if improperly used to undermine, the health of the human body, there is none so constant in our environment as air. At intervals it is necessary to eat and to sleep. At intervals it is equally essential for us to have light; but the use of air goes on from birth to death; completely deprived of it only for a few minutes we die, and it is largely because breathing is so obviously and always essential, because except in definite ill-health it is completely automatic, that few people even give a thought to the question, and most would be disposed to laugh if they were told that there are different ways of breathing, some right and some wrong. Consequently, most people with the inherent perverseness of human nature use one of the wrong ways. Observe, for instance, the way that the first hundred people you meet down any crowded thoroughfare are breathing, and you will find probably that (leaving out of the question those who are evidently out of breath) more than three quarters have their mouths open, and are breathing through them. That is the wrong way. Many of these may have a physical difficulty in getting sufficient air through the nostrils. Some have colds, perhaps, but more have over-large adenoids. Consequently if, when you have no cold at all, you find you cannot get enough air through the nostrils without effort, go straight to a doctor. But probably you can; therefore, breathe through the nostrils. For nature, who, take her all round, is a safe guide to follow, if she clearly indicates something, has provided three passages by which air may reach the lungs. One is the mouth, two are the nostrils. But the mouth (in addition to its sense of taste most conveniently placed there) has the duty of carrying food and drink to the stomach. The chances, therefore, are that the nostrils (in addition to their sense of smell, again most conveniently placed there, a sentinel to challenge the air, as it were, as the taste is a sentinel to challenge the food) were designed to give air to the lungs. And they are not ill-contrived. Witness, to take a horrible but convincing instance, the amount of soot and smuts that are prevented from reaching the lungs if we breathe through the nose during a London fog. The nostrils are a sort of filter, tortuous, averting impurities. On the other hand, many advocates of a sensible idea (as in vegetarianism and total abstinence) are their own foes when they say that only the air warmed by the longer passage is good for the lungs and makes them less liable to catch cold. The reverse is probably the case, since people with delicate lungs are cured of their delicacy or disease in the coldest possible air, if it be dry.