The walls of a stone house, and sometimes of a brick house, are often covered with dampness. This is due to the same cause by which dew is deposited on grass, or moisture on the side of a glass or pitcher filled with ice water and brought into a warm room. The walls become cold, and as stone is a non-conductor of heat, they remain cold for a long time. When the weather changes suddenly from cold to warm, the air becomes filled with moisture, for the warmer the air is the more moisture it will absorb. When this warm air strikes the cold wall, the moisture is deposited on it from the air, which is suddenly cooled by contact with the walls, and as the warm air is continually coming in contact with the walls, the dampness accumulates until it appears like a dew upon them, and pours down in streams at times. It is easily prevented. No plaster should be put directly upon brick or stone, but "furring" strips should be nailed to the wall, and the laths be put on these. Cellars are frequently made very damp in the same way by too much ventilation in warm weather.