The quality of the blood depends almost as much on the air we breathe as on the food we eat; in the course of every two or three minutes, all the blood in the body passes through the endless ramifications of the lungs, for no other purpose than that, during the instant of its passage, it should be acted upon by the oxygen contained in the air which is drawn into the lungs in the act of breathing. But what can happen to the blood in the course of an exposure of so short duration? Just this — the whole character, the very colour, of the blood is changed: it enters the lungs spoiled, no longer capable of sustaining life; it leaves them, a pure and vital fluid. Now, observe, the blood is only fully oxygenated when the air contains its full proportion of oxygen, and every breathing and every burning object withdraws some oxygen from the atmosphere. Hence the importance of giving the children daily airings and abundant exercise of limb and lung in unvitiated, unimpoverished air.