To understand this subject properly, it must be borne in mind, that the body is so constructed as to inhale at every breath about a pint of air. The air is composed of 79 parts nitrogen and 21 parts oxygen. When it is drawn into the lungs, the oxygen is absorbed by the blood, and what we exhale is the nitrogen, mixed with the carbonic acid, formed in the lungs by the union of the oxygen of the air with the carbon of the blood. Now, neither carbonic acid, or nitrogen can support life. Take the oxygen from the air, and then breathe it, and instant death ensues. So, put any animal into carbonic acid alone, and it dies instantly. Thus, every breath of every human being uses up the oxygen in one pint of air, and returns it with only nitrogen and carbonic acid. Let a schoolroom, containing 18,000 gallons of air and twenty scholars, be made perfectly airtight, and in twenty minutes they would all be corpses. The horrible sufferings produced by this process, were once witnessed in Calcutta, where 146 men were driven into a room 18 feet square, with only one small window, and kept there from eight at night till six next morning. Before midnight they all became frantic with agony, fought for the window, choaked each other to death, screamed to the soldiers to shoot them, and thus end their misery; and in the morning only 26 were alive, and these in a putrid fever! _Lessening_ the amount of oxygen in the air by breathing, produces languor, sleepiness, nausea, headache, flushed face, and sometimes palsy and apoplexy. On this subject, the superintendents of the New-York schools make these statements: “Confinement in some of our schoolrooms is _manslaughter_. Our children, shut up in these hot holes, made so by their own breaths, by perspiration, and by a close, overheated stove, lay the foundation for diseases which show no gain except to the physician, and which, in after-life, no riding on horseback, or journeys by sea or land, or southern residence can cure.” Another states, that the uncomfortable condition of the schoolhouses, in his county, is such as to cause much suffering, both mental and bodily, to the children doomed to inhabit their gloomy walls and breathe the tainted air.