Wherever any number of persons live together, the atmosphere becomes poisoned, unless means be provided for its constant change and renovation. If there be not sufficient ventilation, the air becomes charged with carbonic acid, principally the product of respiration. Whatever the body discharges, becomes poison to the body if introduced again through the lungs. Hence the immense importance of pure air. A deficiency of food may be considerably less injurious than a deficiency of pure air. Every person above fourteen years of age requires about six hundred cubic feet of shut-up space to breathe in during the twenty-four hours.[1] If he sleeps in a room of smaller dimensions, he will suffer more or less, and gradually approach the condition of being smothered. [Footnote 1: Where six hundred cubic feet of space is allowed, the air requires to be changed, by ventilation, five times in the hour, in order to keep it pure. The best amount of space to be allowed for a healthy adult is about eight hundred cubic feet. The air which is breathed becomes so rapidly impure, that a constant supply of fresh air must be kept up to make the air of the shut-up space fit for breathing. The following are some amounts of space per head which are met with in practice:-- Artizan rooms 200 cubic feet. Metropolitan Lodging Houses 240 " Poor Law Board Dormitories 300 " Barrack Regulation 60 " The best Hospitals 1,500 to 2,000 cubic feet.] Shut up a mouse in a glass receiver, and it will gradually die by rebreathing its own breath. Shut up a man in a confined space, and he will die in the same way. The English soldiers expired in the Black Hole of Calcutta because they wanted pure air. Thus about half the children born in some manufacturing towns die, before they are five years old, principally because they want pure air. Humboldt tells of a sailor who was dying of fever in the close hold of a ship. His comrades brought him out of his hold to die in the open air. Instead of dying, he revived, and eventually got well. He was cured by the pure air.