The heating effect of compressing air can be well seen when a pneumatic tire is filled. No matter how well the piston of the pump may be oiled, as the density of the air in the tire begins to increase, the pump will grow warm rapidly. This rapid heating cannot be due tofriction, as the pump is not being worked any more swiftly than at first. It is due to the greater compression of the air. As this compression increases, the heating increases, the effect of friction in a well-oiled pump being of small value.
60.Effect of Temperature on the Capacity of the Air to hold Moisture.
Experiment 71. -Take a liter flask and put into it just sufficient water to make a thin film on the inside of the flask when shaken around. Now warm the flask gently, never bringing its temperature near to the boiling point, until the water disappears from the inside and the flask appears to be perfectly dry. Having tightly corked the flask, allow it to cool. The flask appeared dry when warm and on account of having been corked tightly no moisture could have entered it. The air in the flask was perfectly transparent both before and after heating. The film of water around the inside of the flask was taken up by the air when it was warmed but the moisture reappeared when the flask was cooled.
Experiment 72. -Fill a bright tin dish or glass beaker with ice water and after carefully wiping the outside allow it to stand for some time in a warm room. Can water go through the sides of the dish? Does the outside of the dish remain dry? If water collects upon it, from where does the water come? See if the same results will happen if the water within the dish is as warm as or warmer than the air in the room.
Experiment 73. -Partially fill a dish or beaker like that in the previous experiment with water having a temperature a little warmer than that of the room. Gradually add pieces of ice, continually stirring with a chemical thermometer. Note the temperature at which a mist begins to appear upon the outside of the dish. When the mist has appeared, add no more ice but stir until the mist begins to disappear. Note this temperature. Take the average of these two temperatures. This average is probably the temperature at which the mist really began to form. This temperature is called the dew point.
When we wish to dry clothes, we place them in a warm room or inthe sunshine. Soon we find that the water has left the clothes. It musthave gone into the air. It would thus appear that when the temperature of the air is raised, it has the capacity of taking up more moisture than when it is cold. The previous experiment has shown this, and the one in which the dew point was determined showed that when heated air was cooled it deposited moisture.
This property that air has of taking up a large amount of water when heated and giving it out when cooled is the cause of our clouds and rain. If it were not for this there would be no circulation of moisture over the land, no rain, and without rain there could be no vegetation and no animal life. Thus this simple property of the air furnishes the means for the support of practically all the animate life on the earth.
61.Moisture in the Atmosphere.
Experiment 74. -Carefully weigh a dish of water and place it in a convenient place where there is a free access of air. After some hours weigh it again. What causes the change of weight? Try this experiment with a test tube, watchglass and a wide-mouthed beaker under various conditions and in various places.
The atmosphere at all times and under all conditions contains some moisture. When its temperature has been raised, its capacity to hold moisture is increased, but at no place is it so cold that it cannot contain a certain amount of moisture. When water in the solid or liquid condition is exposed to the air, it gradually disappears and is taken up into the air.
If the water surface is large and the temperature high, there is a large amount of evaporation and the water rapidly rises into the air. In the tropics the evaporation from the water surface amounts to perhaps eight feet per year. This means that the energy of the sun lifts about five hundred pounds of water from every square foot of the surface every year. In the polar latitudes the amount of evaporation is perhaps a tenth of that in the tropics.
From every water surface on the globe, however, a large amount of water is evaporated each year. In many places much of the waterevaporated falls upon the same surface from which it came, but a considerable part of it is carried by the winds to other places and falls upon the land surface, furnishing the moisture needed for the land life of the world.
62.Humidity. -The condition of the air as regards the moisture itholds is called its humidity. If the air contains all the moisture it can hold, it is said to be saturatedor to have reached its dew point. The amount of vapor present in the air is called its absolute humidity. The amount of vapor in the air divided by the amount that it would contain if it were saturated is called the relativehumidity. If the air contains
much moisture, its humidity is
CUMULUS CLOUDS.
Typical low level clouds, indicating showers.
said to be high. When air which has a high humidity is cooled, it can no longer hold all the moisture which it previously held, and some mois- ture will be deposited.
The moisture in the air may form into little droplets high above the earth"s surface, making clouds, or these droplets may be near the surface of the earth. In this case we name the moisture fog. If it collects on objects attached to the surface we call it dew.
FOG.
A low cloud formed near the surface of the earth.
By determining the dew point as was done in Experiment 73, and comparing this with tables which have been prepared by meteorologists from many observations, the relative humidity can be readily determined. An instrument for determining the humidity of the air is called a hygrometer (Fig. 62).
