Air is not classed as a food; yet it is the most important food. We can live without the ordinary foods from thirty to forty days, and we can live without water for a few days, but we cannot live without air for more than a few minutes.

Air is the gaseous substance that envelops the earth and forms its atmosphere. It consists almost entirely of the gases oxygen and nitrogen, which are merely mixed and not chemically combined.

An ordinary-sized man is supposed to take through the lungs about two thousand cubic feet of air each twenty-four hours. It is from the air that we secure our greatest supply of oxygen.

Air at sea-level has a pressure of about fourteen and three-fourths pounds to the square inch. It decreases about one-twentieth of a pound per square inch for every ninety feet of altitude. High altitudes cause a quickening of the pulse and breathing. Most people have an idea that there is much danger in going to a high altitude quickly. There is very little discomfort, and almost no danger, to persons in good health.

It is said that, whatever the altitude, the composition of the air is always the same; namely, 21 parts of oxygen, 78.06 of nitrogen, 0.94 of argon, and a trace of carbonic acid.

The only change in the composition of the air in high altitudes is an increase in ozone. Ozone is an allotropic (allotropism: the existence of an element in two or more distinct forms--distinct physical properties).and more active form of oxygen. The variations of the chemical composition of the air do not account for the evil effects experienced in high altitudes; hence the effects must be caused by temperature, pressure, and the action of the sun's rays, which strike more perpendicularly in high than in low altitudes. At an altitude of 4,500 to 5,000 feet the temperature will mark a difference of ten to twelve degrees Fahrenheit in the sun and in the shade. If the bulb of the thermometer be covered with black cotton, the difference will often reach sixty degrees Fahrenheit. This should warn. those in delicate health to prepare themselves with a proper amount of clothing when going into high altitudes. It should not be forgotten, however, that the cold of high altitudes is more tolerable than that of low altitudes, because the air is drier.

The sun, however, does not melt snow unless accompanied with warm air. Black or dark clothes retain the sun's heat and enable the traveler to keep warm in a temperature that would be very uncomfortable at sea level.

The absence of wind and humidity in high altitudes gives comfort, whereas in low altitudes, with a much higher temperature, those who are sick and of low resistance will suffer from the cold.