Page:Elementary Text-book of Physics (Anthony, 1897).djvu/255

§ 225] temperature between the liquid and the vapor formed from it, and the absorption of heat during this process, are explained by supposing that that part of the kinetic energy which measures temperature remains constant, and that the heat is used in doing work against the molecular foi-ces which determine the volume of the liquid. Any further heating of the vapor increases its total kinetic energy and that part of it which measures temperature in nearly the same proportion.

The specific heat of the substance increases when it passes from the solid to the liquid state, and decreases when it becomes a gas. This is explained by the supposition, which many facts render probable, that the kinetic energy of translation of the molecule is greater in the liquid state than in either of the other two states in comparison with the kinetic energy of rotation or of atomic vibration.

The explanation of evaporation which goes on from many solids and liquids at all temperatures has been already given (§ 202); it depends upon the fact that the velocity of some of the molecules is always far greater than the average velocity, and may be sufficient to carry those molecules beyond the range of molecular action.

The hypothesis that the temperature is measured by the kinetic energy of rotation or of atomic vibration is confirmed by its application to Dulong and Petit's law; as it is our purpose to give a general idea of the theory rather than a defence of it, we will not enter upon the discussion of this point.

225. Molecular Velocities and Dimensions.—The formula $$pv = \tfrac{1}{2}mnV^2$$ enables us to calculate $$V,$$ the velocity of mean square, since $$mn$$ is the mass in the volume $$v,$$ and $$p$$ can be measured in absolute units. If we apply the equation to hydrogen under atmospheric pressure, we have p = 1013373 dynes per square centimetre and $$\frac{mn}{v},$$ or the density, = 0.00008954 grams per cubic centimetre, and hence V = 184260 centimetres per second, or a little more than a mile per second. Since for different gases with the same pressure, volume, and temperature $$V^2$$ is inversely as $$m,$$ the velocities in the