3. If a toy balloon containing 2000 ccm. of gas at the earth's surface where the barometer reading is 76 cm., rises to an elevation where the barometer reads 54 cm., the balloon will tend to expand to what volume? Explain. Will it attain this volume? 4. If a gas is compressed, it changes in temperature. How do you explain this? 5. What change in temperature will occur when compressed air is allowed to expand? Explain. 6. Air blowing up a mountain side has its pressure lessened as it approaches the top. How will this affect the temperature? Why? What may result from this change in temperature? Explain. 7. To what pressure must 500 ccm. of air be subjected to compress it to 300 ccm. the barometer reading at first being 75 cm. Explain. [Illustration: FIG. 39.--The air pump.] (3) PNEUMATIC APPLIANCES =60. The Air Pump.=--The air pump is used to remove air or other gases from a closed vessel. It was invented about 1650 by Otto Von Guericke, burgomaster of Magdeburg, Germany. One form of air pump is shown in Fig. 39. _C_ is a cylinder within which slides a tightly fitting piston. _R_ is the vessel from which the air is to be exhausted. _r_ and _u_ are valves opening upward. The action of the pump is as follows: On pushing the piston down, the air in _C_ is compressed. This opens valve _r_ allowing the confined air to escape above the piston. The piston is then raised making the space in _C_ a partial vacuum. The pressure in _R_ now being greater than in _C_, _u_ is pushed up and the air from _R_ rushes into _C_, until the pressure is equalized. On pushing down the piston again, valve _u_ closes and the process is repeated until the pressure in _R_ is no longer able to raise the valve _u_. Some air pumps are so constructed that the valves are opened and closed automatically by the movement of the piston. With these pumps a higher degree of rarefaction can be obtained. Air is often partially exhausted from receivers or vessels by the use of a filter pump or _aspirator_. A stream of water flowing through a constriction causes a reduced pressure, draws in air and carries it away, and thus produces a partial vacuum. See Fig. 40 for a section of the device. [Illustration: FIG. 40.--An aspirator.] =61. The Condensing Pump.=--This is like the exhaust pump except that its valves are reversed. It is used in compressing illuminating gases into cylinders for use in lighting vehicles, stereopticons, Pintsch lights, gas light buoys, etc., and also for compressing air to operate air brakes, pneumatic hammers and drills, and for other uses.