Page:Elementary Text-book of Physics (Anthony, 1897).djvu/104

 to the volume, the limiting or constant velocity reached is less for small than for large bodies. This explains why the fine drops in a fog or cloud fall so slowly that their motion is scarcely noticed, and why shot return to the ground with small velocities, while the velocity of a returning rifle-ball is still considerable.

From considerations based upon the kinetic theory of gases, Maxwell predicted that the coefficient of viscosity in a gas would be independent of its density. This prediction has been verified by experiment through a wide range of densities. For very low densities it has been shown that this law no longer holds true.

76. Theory of Friction.—The friction between solids is due largely, if their surfaces be rough, to the interlocking of projecting parts. In order to slide the bodies over one another, these projections must either be broken off, or the surfaces must separate until they are released. There is also a direct interaction of the molecules which lie in the surfaces of contact. This appears in the friction of smooth solids, and is the sole cause of the viscosity of liquids and gases. That this molecular action is of importance in producing the friction of solids is seen in the facts that the friction of Solids of the same kind is greater than that of solids of different kinds, and that it requires a greater force to start one body sliding over another than to maintain it in motion after it is once started.

77. Fundamental Facts.—If we immerse one end of a fine glass tube having a very small, or capillary, bore in water, we observe that the water rises in the tube above its general level. We also observe that it rises around the outside of the tube, so that its surface in the immediate vicinity of the tube is curved. If we immerse the same tube in mercury, the surface of the mercury within and just outside the tube, instead of being elevated, is depressed. If we change the tube for one of smaller bore, the water rises higher and the mercury sinks lower within it; but the rise or depression outside the tube remains the same. If we immerse the same tube in different liquids, we find that the heights to which