Everyone knows that the breathing of air which has lost little of its due proportion of oxygen is the essential condition of vigorous life and of a fine physique; also, that whatever produces heat, whether it be animal heat, or the heat of fire, candle, gas-lamp, produces that heat at the expense of the oxygen contained in the atmosphere – a bank which is drawn upon by every breathing and burning object; that in situations where much breathing and burning are going on, there is a terrible drain upon this vital gas; that the drain may be so excessive that there is no longer sufficient oxygen in the air to support animal life, and death results; that where the drain is less expensive but still great, animal life may be supported, and people live a flaccid, feeble life in a state of low vitality.