10. Fill a tumbler with water. Place a sheet of paper over the top and invert. The paper clings to the tumbler and prevents the water from escaping. Explain. (See Fig. 35.) 11. Why must a kerosene oil can have two openings in order to allow the oil to flow freely? 12. Explain the action of the modern drinking fountain (Fig. 36). (2) COMPRESSIBILITY AND EXPANSIBILITY OF THE AIR =57. Effect of Pressure on Liquids and Gases.=--Both classes of fluids, liquids and gases, have many characteristics in common. Both are composed of molecules that move freely; hence both _flow_. At any point within a fluid the _pressure is the same in all directions_. Archimedes' Principle applies, therefore, to both liquids and gases. We now come to an important _difference_ between liquids and gases. _Liquids_ are _practically incompressible_. "So much so, that if water is subjected to a pressure of 3000 kg. per sq. cm., its volume is reduced only about one-tenth." Gases show a very different behavior from liquids on being subjected to pressure. They may readily be compressed to a small fraction of their volume as is noticed on inflating a pneumatic tire. A gas has also the _ability to spring back_ to a larger volume as soon as the pressure is released, as when a cork is driven from a pop gun. Not only is compressed air able to expand, but air under ordinary conditions will expand if it is released in a space where the pressure is less. Hollow bodies, animals and plants, are not crushed by atmospheric pressure, because the air and gases contained within exert as much force outward as the air exerts inward. =58. Boyle's Law.=--The relation between the volume and pressure of a gas was first investigated by Robert Boyle in the seventeenth century. The experiment by which he first discovered the law or the relation between the volume and the pressure of a gas is briefly described as follows: [Illustration: FIG. 37 _a_. FIG. 37 _b_. FIGS. 37 _a_ AND 37 _b_.--Boyle's law apparatus.] A glass tube is bent in the form of the capital letter J, the short arm being closed. A little mercury is poured in to cover the bend. (See Fig. 37 _a_.) Since the mercury is at the _same level in both arms_, the pressure in (_A_) is the same as in (_B_). Mercury is now poured into (_A_) until it stands in the long tube at a height above that in (B) which is equal to the height of the mercury column of the barometer. (See Fig. 37 _b_.) The air in (_BC_) is now under a pressure of two atmospheres (one atmosphere is due to the mercury column). On measurement the air in (_BC_) will be found to have just one-half of its original volume. Thus doubling the pressure to which a gas is subjected reduces its volume to one-half. Tripling the pressure, reduces the volume to one-third and so on.