Take these lungs, made up within of branching tubes, these in turn formed by myriads of air-cells, and each air-cell owning its network of minute cells called _capillaries_. To every air-cell is given a blood-vessel bringing blood from the heart, which finds its way through every capillary till it reaches another blood-vessel that carries it back to the heart. It leaves the heart charged with carbonic acid and watery vapor. It returns, if pure air has met it in the lung, with all corruption destroyed, a dancing particle of life. But to be life, and not slow death, thirty-three hogsheads of air must pass daily into the lungs, and twenty-eight pounds of blood journey from heart to lungs and back again three times in each hour. It rests wholly with ourselves, whether this wonderful tide, ebbing and flowing with every breath, shall exchange its poisonous and clogging carbonic acid and watery vapor for life-giving oxygen, or retain it to weigh down and debilitate every nerve in the body.
With every thought and feeling some actual particles of brain and nerve are dissolved, and sent floating on this crimson current. With every motion of a muscle, whether great or small, with every process that can take place in the body, this ceaseless change of particles is going on. Wherever oxygen finds admission, its union with carbon to form carbonic acid, or with hydrogen to form water, produces heat. The waste of the body is literally burned up by the oxygen; and it is this burning which means the warmth of a living body, its absence giving the stony cold of the dead. "Who shall deliver me from the body of this death?" may well be the literal question for each day of our lives; and "pure air" alone can secure genuine life. Breathing bad air reduces all the processes of the body, lessens vitality; and thus, one in poor health will suffer more from bad air than those who have become thoroughly accustomed to it. If weakened vitality were the only result, it would not be so serious a matter; but scrofula is soon fixed upon such constitutions, beginning with its milder form as in consumption, but ending in the absolute rottenness of bone and tissue. The invalid may live in the healthiest climate, pass hours each day in the open air, and yet undo or neutralize much of the good of this by sleeping in an unventilated room at night. Diseased joints, horrible affections of the eye or ear or skin, are inevitable. The greatest living authorities on lung-diseases pronounce deficient ventilation the chief cause of consumption, and more fatal _than all other causes put together_; and, even where food and clothing are both unwholesome, free air has been found able to counteract their effect.