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.
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 continues, until the air in the vessel becomes too much rarefied to lift the delicate valve and make its escape. The vacuum thus produced is by no means so perfect as the “Torricellian vacuum,”[12] the name given to the unoccupied space above the column of mercury in a barometer.
Various substances may be placed in a receiver to show the expansive tendency of air. A piece of wood immersed in a jar of water will throw off thousands of little bubbles. A shriveled apple will become round and plump. The air in an empty rubber bag will often expand so as to fill the receiver. Air in a thin glass vessel, tightly corked, will expand so as to burst the vessel into fragments.
The following simple but useful piece of apparatus can easily be made: Take a pint bottle with a nicely fitted cork, through the cork insert a small glass tube so as to be perfectly air-tight (melted sealing wax is convenient for making tubes or glass tight;) a perforated rubber stopper is better. Let the end of the tube inserted be drawn out in the flame of an alcohol lamp (this is not essential, but will make the experiments more interesting), suck the air from the bottle, close the end of the tube at once with the finger and place it in a glass of water, and a miniature fountain, in vacuo, will be revealed. After performing this pretty experiment remove the tube and reinsert it with the larger end down, having filled the bottle two-thirds full of water. With the lips force a quantity of air into the bottle, upon removing the mouth the water will rise in the tube and fall in a fine spray from the small aperture at the top. This last experiment is particularly interesting, as it is a perfect illustration of a flowing oil well. Closely allied to the expansibility of air is its