Another great difference between the bed-room and the sick-room is, that the sleeper has a very large balance of fresh air to begin with, when he begins the night, if his room has been open all day as it ought to be; the sick man has not, because all day he has been breathing the air in the same room, and dirtying it by the emanations from himself. Far more care is therefore necessary to keep up a constant change of air in the sick room.