238 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME molecules. Of course there are many solids which sublime, producing gases which have no odor. Snow, for example, will "evaporate" without melting. There Is Evidence for the Kinetic-molecular Theory. While it is true that molecules have not been seen, their presence has nevertheless been indicated by the Brownian movement. When nearly submicroscopic particles are viewed with the ultramicroscope, they are seen to move in erratic paths. This constant erratic motion of these very small particles, called the Brownian movement, can be explained by Newton's laws of motion in terms of the bombardment of the particles by molecules of the surrounding medium. It can be shown by a mathematical consideration of the relative masses and velocities of these particles and molecules that the particles should move as they are observed to move. Another line of evidence to support one's confidence in the kineticmolecular theory is the fact that the number of molecules in 22.4 liters r* of any gas under standard conditions of temperature £S) and pressure has been determined by a half dozen or so entirely 1^ independent methods, which give values that /^\ i%. check very closely with each other. This number, i^^M^ J known as Avogadro's number (or constant), is 6.062 X 10^ \^l^r or 606,200,000,000,000,000,000,000. The radiometer, frequently displayed in jewelers' winill W dows, draws the attention of passers-by, because there ^^ is no obvious way to account for the constant turning of ^1 its vanes in its airtight glass bulb. The air within the II glass bulb is partially evacuated, with the result that ^^^^^ ""^ tiG. 74. Ihe there is little resistance offered to the motion of the spindle and the attached four vanes as it rotates on its (;jeiicate bearings. One side of each vane is painted with lampblack, while the other side is highly polished. Inasmuch as highly polished or light-colored surfaces reflect light while dark surfaces absorb it, the black side of each vane is heated more than the polished side when the radiometer is exposed to light. Why should this unequal heating of the vanes cause the spindle to turn? Here we have to resort to the kinetic-molecular theory for an explanation. The air molecules which strike the warmer surface acquire a greater velocity of motion and therefore rebound more violently than the molecules on the cooler sides of the vanes. This recoil is sufficient to rotate the spindle about its axis. The diffusion of molecules within metals shows that there are mole-
THE KINETIC-MOLECULAR THEORY 239 cules in such solids. The surfaces of small blocks of metals may be made so true and may be polished so highly that when placed next to each other they will stick together ; both blocks can be lifted as a unit by picking up either block. This intimate contact is supposed to bring enough molecules close enough together to bring about considerable attraction between them. The presence of moving molecules is demonstrated by the fact that such blocks of different metals as lead and gold, when left in contact with each other for some time and then tested, show that appreciable amounts of gold have diffused from the gold blocks into the lead blocks and vice versa. STUDY QUESTIONS 1. State the main points of the kinetic-molecular theory. 2. What does the kinetic-molecular theory explain? 3. Why are the sides of a football kept distended after it is blown up? 4. Explain Boyle's law in terms of the kinetic-molecular theory. 5. Why is it difficult to compress solids? 6. Explain in terms of the kinetic-molecular theory why a tire is more likely to blow out on a hot day than on a cold day. 7. Show how the kinetic-molecular theory explains: (c) the states of matter, (fe) gas pressure, (c) why a liquid has surface tension, (d) the true nature of evaporation, (e) what the boiling-point is, (/) what the melting-point is, (g) compressibility, (h) diffusibility, (i) liquefaction, (j) why gases expand when heated. 8. Account for the following phenomena in terms of the kinetic-molecular theory: (a) the effect of temperature and pressure changes on the viscosity of a liquid, (6) the difference between the two classes of solids, (c) solids do not take the shapes of vessels in which they are placed, while liquids do, (d) the cooling of a gas on expansion, (e) the effect of temperature and pressure on the volume of a gas. 9. Explain why liquids and solids are less compressible than gases. 10. Mention at least one line of evidence which supports the kinetic-molecular theory.