The air has a strong attraction for water, and always holds a certain quantity as invisible vapor. The warmer the air, the more moisture it demands, and it will draw it from all objects within reach. The air holds water according to its temperature. Thus, at fifty-two degrees, Fahrenheit's thermometer, it holds half the moisture it can sustain; but at thirty-six degrees, it will hold only one eighty-sixth part. The earth and all plants and trees are constantly sending out moisture; and when the air has received all it can hold, without depositing it as dew, it is said to be _saturated_, and the point of temperature at which dew begins to form, by condensation, upon the surface of the earth and its vegetation, is called the _dew-point_. When air, at a given temperature, has only forty per cent of the moisture it requires for saturation, it is said to be dry. In a hot summer day, the air will hold far more moisture than in cool days. In summer, out-door air rarely holds less than half its volume of water. In 1838, at Cambridge, Massachusetts, and New-Haven, Connecticut, at seventy degrees, Fahrenheit, the air held eighty per cent of moisture. In New Orleans, the air often retains ninety per cent of the moisture it is capable of holding; and in cool days at the North, in foggy weather, the air is sometimes wholly saturated. When air holds all the moisture it can, without depositing dew, its moisture is called 100. When it holds three fourths of this, it is said to be at seventy-five per cent. When it holds only one half, it is at fifty per cent. When it holds only one fourth, it is at twenty-five per cent, etc. Sanitary observers teach that the proper amount of moisture in the air ranges from forty to seventy per cent of saturation. Now, furnaces, which are of course used only in winter, receive outside air at a low temperature, holding little moisture; This it sucks up, like a sponge, from the walls and furniture of a house. If it is taken into the human lungs, it draws much of its required moisture from the body, often causing dryness of lips and throat, and painfully affecting the lungs. Prof. Brewer, of the Scientific School of New-Haven, who has experimented extensively on this subject, states that, while forty per cent of moisture is needed in air to make it healthful, most stoves and furnaces do not, by any contrivances, supply one half of this, or not twenty per cent. He says most furnace-heated air is dryer than is ever breathed in the hottest deserts of Sahara.