It has been shown how the air is heated by _convection_, or changing contact. It is thus the atmosphere is warmed, not by the rays of the sun passing through it, but by contact with the earth and other objects which have been warmed by radiated heat from the sun. The lower stratum of air being thus warmed, becomes lighter, and ascends, giving place to the cooler and heavier air. This process continues, so that the warmest air is always nearest the earth, and grows cooler as height increases. The air has a strong attraction for water, and always holds a certain quantity as an invisible vapor. The warmer the air the more water it demands, and will draw it from all objects it can reach. When air cools, it deposits its invisible moisture as dew. When the air has all the water it can hold, it is said to be _saturated_; and when it cools so as to begin to deposit moisture, it is called the _dew point_. When air holds all the moisture it can sustain, its moisture is said to be at 100 per cent.; when it holds only one-half as much as its temperature demands, it is said to be at 50 per cent.; and when it holds three-fourths of what its temperature requires, it is at 75 per cent.; and when only one-fourth, it holds 25 per cent. In summer, outdoor air rarely holds less than half its _volume_ of water; that is, a quart of air usually holds as much as a pint of invisible vapor. In 1838, at Harvard and Yale, at 70° Fahrenheit, the air held 80 per cent. of moisture; at New Orleans it often holds 90 per cent.; at the North, in fogs, the air often holds all it can, or is saturated—that is, holding 100 per cent. Thus it appears that the hotter the air, the more water is demanded by it for invisible vapor, and this it takes from all around. Professor Bremer, of Yale College, states that 40 per cent. of moisture is needed to make air healthful. Now furnaces receive cold air containing little invisible moisture, and by heating it a demand is created for much more. This is sucked up, as by a sponge, from walls and furniture, and especially from the lungs and capillaries of our bodies, thus causing dryness and sometimes inflammation of lips, nose, eyes, throat, and lungs. Experiments prove that while 40 per cent. of moisture is needed for health, furnace-heated air rarely has as much as 20 per cent., even when a few quarts of water are evaporated in the furnace chamber. Thus the inmates of the house breathe dryer air than is ever breathed in the hottest deserts of Sahara.