=2. Vegetable growth.= Plants absorb carbonic acid (which is composed of carbon and oxygen) through their leaves, and give back oxygen to the air, retaining the carbon for their own nourishment. There is thus a constant interchange between animals and vegetables, the former exhaling carbonic acid and appropriating oxygen, and the latter appropriating carbonic acid and exhaling oxygen. The small percentage of carbonic acid always found in the air is, therefore, essential to vegetable life, while harmless to animals. It is necessary, for the proper purification of a contaminated atmosphere, that it should be largely diluted with fresh air. Hence arises the need of the constant change of air in dwellings. Air expands when heated and so becomes lighter. Local differences of temperature, created by natural and artificial means, therefore bring about currents in the atmosphere, the cooler and heavier column of air always descending, and the warmer and lighter always rising. This fact is taken advantage of in ventilation. It has been estimated that, to keep the air pure, three thousand cubic feet of fresh air per hour are required for a male adult, and that a sleeping-room should contain at least twelve hundred cubic feet of air-space for each occupant. When the temperature of the external air is such that the doors and windows can be constantly open, they afford the best means of ventilation for dwellings. An exposure to draughts, however, is dangerous to many persons, and it is desirable, therefore, in cooler weather, to devise means of admitting fresh air without creating a draught. At a temperature of 60°, a draught is perceived when the air moves at a higher rate of speed than three feet a second. Now it is obvious that a draught may be rendered harmless if the entering current of air is guided in such a direction as not to strike the occupants of a room. This is accomplished simply and cheaply by either of two devices: If the lower sash of a window is raised a few inches (say four), and the space between the bottom of the sash and the window-sill is filled by an accurately fitted board, there will be a space between the panes of the two sashes, through which air will enter, spouting upward toward the ceiling and not falling until its momentum is so much diminished that it will not be felt as a draught. The other plan is to make the upper portion of the upper sash movable, so that it can be tilted inward at such an angle as to direct the entering current upward (essentially the Sherringham valve, though this is made of iron, with side-cheeks to prevent a lateral outflow of air). There are various patent apparatuses for the admission of fresh air through windows without draught, but they are mostly modifications of the methods above mentioned.