A flask containing water is closed by a rubber stopper through which pass the stem of a glass funnel and a bent glass tube that has been drawn out to a small opening (_J_). The funnel has cemented in its top an inverted porous clay jar (_C_), over the top of the latter is placed a beaker (_B_). A piece of flexible rubber tubing (_H_) leading from a hydrogen generator is brought up to the top of the space between the jar and the beaker. When hydrogen gas is allowed to flow into the space between _C_ and _B_, the level of the water in _W_ is seen to lower and a stream of water runs out at _J_ spurting up into the air. On stopping the flow of hydrogen and removing _B_, the water falls rapidly in _J_ and bubbles of air are seen to enter the water from the tube. (The foregoing steps may be repeated as often as desired). This experiment illustrates the fact that the molecules of some gases move faster than those of some other gases. Hydrogen molecules are found to move about four times as fast as air molecules. Hence, while both air and hydrogen molecules are at first going in opposite directions through the walls of _C_, the hydrogen goes in much faster than the air comes out. In consequence it accumulates, creates pressure, and drives down the water in _W_ and out at _J_. On removing _B_, the hydrogen within the porous cup comes out much faster than the air reënters. This lessens the pressure within, so that air rushes in through _J_. This experiment demonstrates not only the fact of molecular motion in gases but also that molecules of hydrogen move much faster than those of air. (This experiment will work with illuminating gas but not so strikingly.) Careful experiments have shown that the speed of ordinary air molecules is 445 meters or 1460 ft. per second; while hydrogen molecules move at the rate of 1700 meters or 5575 ft. or more than a mile per second. =16. Expansion of Gases.=--Gases also possess the property of indefinite expansion, that is, if a small quantity of gas is placed in a vacuum, the gas will expand immediately to fill the entire space uniformly. This is shown by an experiment with the air pump. On raising the piston the air follows instantly to fill up the space under it. As the air is removed from the receiver of an air pump the air remaining is uniformly distributed within. =17. How Gases Exert Pressure.=--It is further found that air under ordinary conditions exerts a pressure of about 15 lbs. to the square inch. In an automobile tire the pressure may be 90 lbs. and in a steam boiler it may be 200 lbs. or more to the square inch.