[18] This nitrogen, though of no value for breathing, is of great value as a food, forming, as we have seen, an important part of all meats, or proteins, which build the tissues of our bodies. It can, however, be taken from the air only with great difficulty, by a very roundabout route; the bacteria of the soil eat it first, then they pass it on as food to the roots of plants; animals eat plants, and we eat the animals, and thus get most of our nitrogen. CHAPTER XIV HOW TO KEEP THE LUNG-BELLOWS IN GOOD CONDITION THE NEED OF PURE AIR Free Air is Pure. As air, in the form of wind, actually sweeps all outdoors, day and night, it clearly is likely to pick up a good many different kinds of dust and dirt, which may not be wholesome when breathed into our lungs. Fortunately, nature's great outdoor system of purifying the air is almost perfect, so that it is only when we build houses and shut in air from the great outdoor circulation, that "dirt" that is really dangerous begins to get into it. Caged air is the only air that is dangerous. Free-moving air is always perfectly safe to breathe any hour of the day or night, or any season of the year. Shut-in and Stagnant Air is Foul. This restless air-gas cannot be stored outside of the body, any better than it can be inside. For one thing, it is too bulky; and for another, it begins to become impure in various ways, as soon as it is shut up. It is the most unmanageable food that we "eat," for we can neither cook it nor wash it like solid food, nor filter it nor boil it like water, except on a very limited scale. We can do nothing to it except to foul it, which we do with every breath that we breathe, every fire that we make, every factory that we build. Our only chance of safety, our only hope of life, is to connect every room and every corner of those little brick and mortar boxes, those caged sections of out-of-doors, that we call houses, with nature's great system of air supply, "All Outdoors." Fortunately, the only thing needed to make the connection is to open a window--no need to send for a plumber or put in a meter, and there is no charge for the supply after connections have been made.