Carbonic acid in the air of a classroom is an index of impurity. External air has about three parts of carbonic acid in 10,000 parts of air, and above seven parts in 10,000 is injurious. Each person exhales about fourteen cubic feet of carbonic acid gas in an hour. There is no easy method of determining the quantity of carbonic acid gas present in a room, and we must therefore arrange the ventilation so that about 3000 cubic feet of fresh air an hour will be supplied to each person in the house. Beside carbonic acid there are other impurities in house air, as dust, micro-organisms of disease, exhalations from bodies, sewer gas, and the like, which accumulate and do injury when the ventilation is defective. If every person in a house has 1000 cubic feet of air space, natural ventilation will suffice ordinarily, but artificial ventilation is needed in schoolrooms and dormitories. The subject of ventilation can not be satisfactorily discussed in a short article, and those that are interested in school building should leave the matter to a competent architect, or study books and articles like J. S. Billings' _Ventilation and Heating_, Pettenkofer's _Ueber Luft in den Schulen_, and Kober's article on House Sanitation in the _Reference Handbook of the Medical Sciences_. {207} The proper heating of a schoolroom is a matter so generally understood that there is no need for special remark here, except this, that provision for proper humidity in the heated air is commonly neglected. Cheap water-closets do not save money--they get out of order too easily. The pan, valve, and plunger hoppers are not to be tolerated. The only kind to use are short-hopper closets with a trap that opens into the soil-pipe above the floor. These may have valve-lifters attached to the seats, because children forget to flush the hoppers. The ventilation of the water-closets should be separate from that of the main building. In country places where vaults are used, there should be a supply of dry loam kept, and enough of this to cover the fresh contents should be thrown into the vaults every evening. Children are seemingly always thirsty, and they should be allowed to have all the drinking water they want if the source is free from typhoid germs and infection by organic matter. Common cups are an abomination, and a prolific cause of contagious diseases. Each child should have its own cup. The rules for desks and seats for children are these: 1. The height of the seat should be about two-sevenths of that of the body. 2. The width of the seat should be about one-fifth of the length of the body, or three-fourths the length of the thigh. Do not keep unfortunate little children's feet dangling all through their school years to save a few pennies on school furniture. 3. The seat should slope downward a little toward the back, be slightly concave, and have rounded edges in front. 4. There must be a back-rest.