Experiments seem to show that when the amount of carbon dioxid in the air has doubled, that is, when the expired air mixed with the air in the room has increased the proportion of carbon acid from four parts in ten thousand to eight parts in ten thousand, that the air is seriously affected, and that such ventilation ought to be provided that no greater amount than this could occur. This is such a condition that the room smells "close" or stuffy to a person coming in from outdoors, indicating organic emanations as well as an excess of carbonic acid gas. The question then is: how may this condition be avoided in an ordinary house, or in an ordinary stable, because the health of the cattle on a farm, judging at least by the character of the buildings provided, is quite as important as the health of the farmer's family. We must take it for granted that no such elaborate schemes are possible as in public buildings or schools, where fans are provided, either to force air into the several rooms or else to suck it out. The ventilation of the house must be more simple and easily adjusted and must depend on the principle of physics that warm air rises and that if the warm air of a room is to be removed, air must in some way be supplied to take its place. The two essentials for ventilation are opportunity for the ingress and the egress of air--ingress for fresh air and egress for polluted air. _Fresh-air inlet._ In the construction, of a dwelling house, special and adequate preparation for the admission of fresh air is seldom provided, so that the existing openings must be used for the purpose. This means that in the summertime an open window will furnish all the fresh air which a room receives and, when the temperature of the outside air is approximately that of the living room, such provision is ample and satisfactory. But in the wintertime, when the outside air is cold, the average person will prefer to suffer from the bad effects of impure air rather than admit cold air which may cause an unpleasant draft. [Illustration: FIG. 14.--Letting in fresh air.] One of the simplest and best methods of providing an inlet for fresh air, without at the same time allowing blasts of wind to enter the room, is to fasten in front of the lower part of the window a board which shall just fill the window opening; then, raising the lower sash a few inches will allow fresh air to enter both at the bottom, where the board is placed, and at the middle of the window between the sashes (see Fig. 14). Persons sitting close by a window thus arranged may feel a draft even under these conditions, since the cold air thus admitted will sink at once to the floor and then gradually rise through the room to the ceiling, but unless one sits too near the window, this is an admirable method of admitting fresh air.