“As a matter of fact, the air has nothing to do with it, for it has always been clearly observed that the presence of water vapor in any given space is independent of the presence or absence of air in the same space. The amount of aqueous vapor which a space contains depends entirely upon the temperature and not upon the presence of the air.”—Rosenau in _Preventive Medicine and Hygiene_. The higher the temperature the greater the amount of water vapor in one cubic foot of air at a temperature of 10° F., while at 100° F. there would be 19.1 grains at complete saturation. Since increased temperature increases the amount of aqueous vapor and this aqueous vapor in turn absorbs heat, we thus see a reciprocal action of the aqueous vapor upon the temperature. =Absolute humidity= is all of the water vapor that may be contained in the air at a given temperature. =Relative humidity= is the difference between the amount of water vapor that must be contained in the air at a given temperature to reach absolute saturation and the amount actually contained in the air at that same temperature. If the relative humidity of the air in a room becomes as high as 85% the moisture will begin to condense and form on the walls and objects. This makes the room damp and interferes with the ventilating and heating of the room. There is less water vapor contained in the air at high altitudes, the air being cooler. A large amount of rainfall does not necessarily produce an increase in the relative humidity. That is to say, a country with a very high average of rainfall is not necessarily a damp country so far as the atmosphere is concerned. Cold Dry Air Cold dry air is exhilarating and tends to quicken metabolism in the body, while warm damp air is depressing and tends to retard metabolism. The body possesses great possibilities of adaptation to the varying degrees of temperature and humidity through the action of Innate Intelligence. With the aid of the educated mind in bringing about adaptation in the way of clothing, for example, it is possible to increase the range of temperature and humidity to which the body may be adapted. Due to the fact that heat is being constantly formed in the body by the different processes that are carried on within, it naturally follows that this heat must be carried out of the body or it will accumulate and result in harm to the tissues, producing what is known as heat stroke. This heat dissipation is greatly influenced by the humidity, or in other words, the amount of water vapor in the air. The temperature of the air also has some influence on heat dissipation. Cold air is made to feel colder by an increase in the amount of moisture while warm or hot air is made hotter by increasing the moisture. The reason for this is that the moisture in the cold air favors heat conduction, hence draws the heat from the body at a more rapid rate than is normal, while the moisture in the hot air hinders evaporation.