86a Handbook of Nature-Study 



and during cloudy weather than during clear weather. Air containing 

 more than 0.06% of carbon dioxide is not fit to breathe, not because air 

 loaded with carbon dioxide is poisonous, but because it excludes the oxygen 

 and thus produces death by suffocation. It is considerably heavier than 

 air, and in certain localities, where it is emitted from the ground, accumu- 

 lates in low places in such quantities as to suffocate animals. Death's 

 Gulch, a deep ravine in Yellowstone Park, and Dog's Grotto near Naples, 

 are examples. At the latter place, the gas, on account of being heavier than 

 air, lies so close to the ground that a man, standing erect, will have no 

 difficulty in breathing, while a dog will die of suffocation. It also accumu- 

 lates in unused wells, cisterns and mines, and can usually be detected by 

 lowering a lighted candle. If carbon dioxide is present in large quantities, 

 the candle will be extinguished because of the lack of oxygen to support 

 combustion. 



Although carbon dioxide forms but a small proportion of the atmos- 

 phere, it is a very important element in plant life. Animals consume 

 oxygen and exhale carbon dioxide, while plants take in carbon dioxide and 

 give off oxygen ; thus, the amount of these gases in the atmosphere is main- 

 tained at an equilibrium. Plants, through their leaves, absorb the carbon 

 dioxide, which is decomposed by the sunlight, returning the oxygen free 

 into the air, while the carbon is used to build up plant tissue. 



Other Gases 



Argon, on account of its resemblance to nitrogen, was not discovered 

 until 1894, having been included with the nitrogen in all previous analyses 

 of air. It constitutes about i% of air by volume. Krepton, neon and 

 xenon exist in minute quantities and have some interest chemically, but 

 little for the meteorologists. Helium and hydrogen probably exist at great 

 elevations in the atmosphere. 



Water Vapor 



The vapor of water in the atmosphere varies from about one per cent 

 for arid regions to about five per cent, of the weight of the air for warm, 

 humid regions. It is a little over one-half as heavy as air and moist air is, 

 therefore, lighter than dry air ; but the increase of moisture near the center 

 of cyclones has only a slight effect in reducing the pressure. The amount of 

 vapor decreases very rapidly with elevation, and probably disappears at an 

 elevation of five or six miles above the surface. The amount of water in 

 the form of vapor that can exist in the atmosphere increases with the 

 temperature, being .54 grains Troy per cubic foot at zero temperature and 

 14.81 at 90. < When the air has taken up all the moisture it can contain 

 at a given temperature it is said to be saturated. 



The dewpoint is the temperature at which saturation occurs. If the 

 air is saturated, the temperature of the air and the dewpoint will be the 

 same, but if the air is not saturated the dewpoint will be below that of the 

 air. 



Relative humidity is expressed in percentages of the amount necessary 

 to saturate. If the air contains one-half enough vapor to saturate it, the 

 relative humidity will be 50%; if one-fourth, enough to saturate, 25%; if 

 saturated 100% etc. 



The absolute humidity is the actual amount of water in the form of 

 vapor in the air, and is usually expressed by weight in grains per cubic foot 



