For floors of cellars or barns, the dirt should be evened off and tamped and then the cement concrete should be spread evenly over it, and tamped just enough to bring the water to the surface. When partially dry, a better finish is obtained by lightly troweling the concrete. In a cellar or barn, it is not necessary to divide up the area into squares or blocks as is done with sidewalk work, but the entire area may be laid in one piece. In order to keep the surface level, however, it may be found convenient to lay down pieces of 2" x 4" scantling, the tops of which shall be on the desired level of the finished floor. By filling in behind these scantlings, which can be moved ahead as the filling progresses, the exact level desired can be obtained. Usually four inches thick will be a proper depth of concrete for this purpose. CHAPTER IV _VENTILATION_ The average individual breathes in and out about eighteen times a minute, taking into his lungs the air surrounding him at the time and expelling air so modified as to contain large amounts of carbonic acid, organic vapor, and other waste products of the lungs. The volume of air taken in is about the same quantity as that expelled and amounts to eighteen cubic feet per hour. Fortunately, the air expired at a breath is at once rapidly diffused throughout the surrounding atmosphere, so that, even if no fresh air were introduced, the second breath inhaled would not be very different from the first. But after a certain length of time the air becomes so saturated with the waste products of the lungs that it is no longer fit to breathe, and it is evident that in order to keep the air in a room so that it can be taken into the lungs with any reasonable degree of comfort, there must be a continual supply of fresh air admitted with a proper provision for discharging polluted air. If this is not done, there is, so far as the lungs are concerned, a process established similar to that which is occasionally found when a village takes its water-supply from a pond and discharges its sewage into the same pond. Not long ago, the writer found in the Adirondacks a hotel built on the side of a small lake which pumped its water-supply from the lake, and discharged its sewage into the same lake only a few feet away from the water intake. That the hotel had a reputation of being unhealthy, and that it had difficulty in filling its guest rooms, is not to be wondered at, and yet individuals will treat their lungs exactly as the hotel treated its patrons. _Effects of bad air._ In order to establish a proper relation between the amount of impurities diffused through the air and the physiological effect on individuals breathing that air, certain observations have been noted and certain experiments have been made which prove without question the injurious effect of vitiated air.