1. =Plants=, by virtue of the chlorophyll contained in their green parts, absorb carbonic acid from the atmosphere, liberating oxygen in an active condition. In addition, ammonia and nitrous and nitric acids are dissolved from the air by rain-water, and assimilated by plants. During the night plants only give off carbonic acid. 2. =The Fall of Rain= clears the atmosphere of any solid particles contained in it, the impurities being transferred to rain-water which generally contains an appreciable amount of ammonia as well as other impurities. Rain not only washes and purifies the air, but by washing the ground, diminishes dust, and prevents its escape into the air. It is the great natural scavenger. 3. =Ventilation=—that is, the interchange of pure and impure air, is constantly being effected. Before entering on the details of ventilation, we must consider the _physical causes_ at work which tend to purify the air, apart from all artificial contrivances. These are three in number—namely, diffusion, winds, and differences of temperature of masses of air. (1) =Diffusion= causes the rapid mixture of gases placed together. Every gas diffuses at a certain rate—namely, inversely as the square root of its density. In any room which is not air-tight, diffusion is constantly occurring, air passing in and out at every possible point. Through chinks and openings in the carpentry-work of a room, the air diffuses rapidly. Bricks and stone commonly allow air to pass through them; diffusion occurs to a slight extent even if the wall is plastered, but very little through paper. Diffusion alone is quite insufficient to purify a room under ordinary circumstances; and solid particles including the organic matter evolved from the skin and lungs, not being gaseous, are unaffected by it. To remove these, the room must be periodically flushed with air, and washing of all dirty surfaces must be carried out. Diffusion sometimes produces evil results, when the sanitary arrangements of a house are bad. If there is a leakage of sewage under the kitchen floor, the foul gases from it diffuse upwards; occasionally foul air diffuses from the dust-bin through the wall into the rooms of a house. These results are helped by the fact that the internal temperature of a house is commonly higher than the external.