In addition to all this, the air is filled with dust brought in and kept astir by many busy feet; with ashes floating from the stove or furnace; and especially with chalk dust. The modern method of teaching requires a large amount of blackboard work, and the air of the schoolroom is thus loaded with chalk particles. These collect in the nasal passages, and the upper part of the larynx, and irritate the membrane, perhaps laying the foundation of catarrh. The usual schoolroom atmosphere bears in the pupils the natural fruit of frequent headaches, inattention, weariness, and stupor; but in the teacher its frightful influence is most apparent. His labor is severe, his worry of mind is constant, and, when he finishes his day's work, he is generally too tired to take proper physical exercise. He consequently labors on with impaired health, or is forced to abandon his profession. Instead of six hundred feet of space being allowed for each pupil, as perfect ventilation demands--the lowest estimate being two hundred and fifty feet--often not over one hundred feet are afforded. Instead of two thousand cubic feet of fresh air being supplied every hour for each person, and as much foul air removed, which, all physiologists assert, is needed for perfect health, perhaps no means of ventilation at all are provided, and none is secured except what an occasionally opened door, or the benevolent cracks and chinks in the building furnish the suffering lungs. [Footnote: Imagine fifty pupils put into a class room thirty feet long, twenty-five feet wide, and ten feet high. This would generally be considered a very liberal provision. Such a room contains seven thousand five hundred cubic feet of air. But it furnishes only one hundred and fifty feet of space for each pupil. Allowing ten cubic feet of air per pupil each minute, in fifteen minutes after assembling, the entire atmosphere of the room is tainted, and unfit to be rebreathed. The demand of health is that at least one thousand five hundred cubic feet of pure air should be admitted into this room every minute, and as much be removed.]