Partially fill a glass flask with water, heat it until steam begins to escape, then remove the lamp and insert a stopper, the boiling will cease. Now pour cold water upon the flask, and the water within begins again to boil vigorously. The cold water condenses the steam, creating a partial vacuum, thus relieving the heated water from pressure, and it boils at a lower temperature than 212°. This illustrates the famous culinary paradox that “cold water will make hot water boil.” The buoyancy of substances in air depends upon the same principle that determines their buoyancy in liquids. It will be proportioned to the amount of air which they displace. It is correct to say that a balloon rises because the air is heavier, and therefore pushes under the balloon and forces it up; or, that it rises because it displaces more than its own weight of air. Thistle-down may be compressed so that it will fall like shot. The resistance offered by the air to the fall of bodies led men long to hold to the fallacy that the rapidity of the descent of falling bodies was proportioned to their weight. This error was at length exploded by Galileo in his interesting experiment on the leaning tower of Pisa.[9] In a vacuum, a cannon ball and a feather fall in the same time. [Illustration: AIR PUMP.] COMPRESSIBILITY. The statement that air can be expanded involves the counter-truth that it may be compressed. Mariotte[10] announces the law as follows: Doubling the pressure upon a given amount of gas will halve the space it occupies, and double its expansive energy. The application of this principle in one form gives us the air-gun.[11] If the air in a gun-barrel forty inches long were compressed into the space of half an inch it would press with eighty times its previous force, or with a power equal to twelve hundred pounds to the square inch. Compressed air is often used as a power in mines and excavations, and its advantages are many; it was so employed in the Hoosac tunnel. Though the engine that compressed the air was three miles away, the loss from friction was very slight, and the air, having performed its work in driving the drill, was then liberated to purify the atmosphere of the tunnel and expel noxious gases which accumulated from continuous blasting. The apparatus for compressing air is called a condenser. It consists essentially of a cylinder and piston, with a valve in the bottom of each, opening downward. A precisely opposite arrangement of valves is found in the air-pump, a machine for exhausting air from a given space, usually a receiver. As the piston is raised in removing the air, the valve closes, and the air is thus forced out of the cylinder; the air in the receiver then expands, opens the valve at the bottom of the cylinder, and rises into it; as the piston descends its valve is opened; rising, it again removes the cylinder full of air; the air in the receiver again expands, opens the lower valve, and so