would be a rising current of hot air and a current of colder air creeping in to take its place. The winds of the earth are due to this property of air. It is this tendency of heated air to rise that makes hot air furnaces useful for heating houses. Valleys are generally colder than the surrounding hillsides, so that delicate crops can be grown successfully on the hillsides although those in the valley are frost bitten.
55.Pressure of Air.
Experiment 62. -Use a convection apparatus or take a tight chalk box and in two places on the top punch holes in a circle not quite as large as theFig. 53.
bottom of a lamp chimney. Place a small lighted candle at the center of one of the circles of holes and a lamp chimney, tightly sealed to the box, about each circle. Hold a smoking piece of paper above the chimney which does not inclose the candle. (If a pane of glass is put into one of the vertical sides of the box, better observations can be made.) What happens? Put out the candle and carefully heat the chimney with a Bunsen burner. Is there the same action as before? Why is it that sparks rise from a fire? What is meant by the draft of a stove? Why inorder to ventilate a room is it best to open a window at the top and bottom?
Experiment 63. -If a tin can with a tightly fitting screw cap can be easily procured, boil a little water in it, having the screw cap open so that the steam can readily escape. While the water is still strongly boiling, quickly remove from the heat and tightly cork. Be sure not to cork before removing entirely from the heat. Set the tin thus corked upon the desk and observe. What happens as the steam condenses? Why?
Experiment 64. -By means of an air pump exhaust the air from a pair of Magdeburg hemispheres. Now try to pullthe hemispheres apart. It cannot be done
as easily as before the air was exhausted.
Why?
Fig. 54.
Fig. 55.
Experiment 65. -Fill a glass tumbler even full of water and press upon it a piece of writing paper. Be sure that the paper fits smoothly to the rim of the tumbler. Take the tumbler by its base and carefully invert it over a pan. Does the water fall out? If not, why not? While the tumbler is in the invertedposition, insert the point of a pencil between the paper and the rim of thetumbler. What happens?
Anything that has weight must exert pressure upon the surface upon which it rests. The air has been found to have weight, therefore it mustexert pressure at the surface of the earth. Air is a gas and its particles easily move over each other, therefore this pressure is exerted equally in all directions. No one feels the pressure, however, because the air is within us as well as about us. Those that have measured this pressure find that it is about fifteen pounds to the square inch at sea level. If two egg shells from which the insides had been removed, one of them with the holes left in it and the other completely sealed, were sunk to a considerable depth in water, which one would be crushed and which one would not? This illustrates why we are not crushed by the pressure of the air upon us.
56.Decrease of Volume due to Pressure.
Experiment 66. -In a Mariotte"s tube cause about a centimeter of mercury in the short arm to balance the same amount in the long arm. The pressure inside the short tube will then be equal to that outside the long tube and will be that of the air upon the day of the experiment. The short arm will now be sealed with mercury so that no air can get in or out. Pour mercury into the long arm. The air in the short arm will be gradually compressed and will occupy less and less space. If we remember that the pressure upon the air in the short arm is the air pressure of the day plus the height that the mercury column in the long arm exceeds that in the short arm, we can show by careful measurement that the volume of the air decreases just as the pressure increases.
As was seen in Experiment 1, the volume of the air can be very much decreased by pressure, but whenFig. 56.
the pressure is removed, it regains its original volume. It cannot be told from this experiment whether the volume of the gas decreases as the pressure increases or whether it decreases much more rapidly when first pressed upon than afterward. This can be best shown by the use of the Mariotte"s tube as in Experiment 66. But if the bicycle pump is a good one, it will answer the question of the rate of decreasequite accurately. It is found that the volume decreases directly as the pressure increases.
Fig. 57.
57.Barometers. -As the measurement of the atmospheric pressure is of great importance in the study of atmospheric conditions, it is necessary to have an instrument by which these measurements can be readily made. An instrument designed for this purpose is called a barometer. There are twokinds of barometers in common use, called the mercurial and theaneroid.
Experiment 67. -(Teacher"s Exp.) Take a thick-walled glass tube of about 1/2 cm. bore and about 90 cm. long and slip tightly over the end of it about 10 cm. of a thick-walled flexible rubber tube 30 cm. in length. Firmly secure the rubber tube to the glass tube by winding tightly around them many turns of string, making it impossible for the rubber tube to slip or admit air. Completely close the rubber tube with a Hoffman"s screw just beyond the place where it leaves the glass tube. Placing this closed end in a large dish so as not to waste any mercury, fill the glass tube with mercury. Place the thumb over the open end of the tube and invert it in a cup of mercury. If the connections were made tight, the mercury will not fall far below the end of the glass tube. The air pressure keeps the mercury up. This is a simple form of barometer.
While the tube is still standing in the mercury cup take another glass tube similar to the first and attach it to the open end of the rubber tube in the same way as the first was attached. Place the free end of this tube in a dish of colored water and gradually open the Hoffman"s screw. The water rises in the tube. Why? What is meant by sucking water up a tube?