Page:Elementary Text-book of Physics (Anthony, 1897).djvu/254

240 become rotatory and they are free to glide past each other in the mass. We can explain the constancy of temperature during melting, and the absorption of heat, by assuming that that portion of the energy which measures temperature remains constant, and that the heat is used in doing work against the molecular forces which determine the direction of the molecules in the solid and in giving the molecules increased velocity of translation. Such a change as is here described, in which the energy received by the molecule does work against the forces acting on it and gives it greater velocity as a whole, while the mean energy of vibration which it had at first is equal to the mean energy of rotation which it acquires, has been shown by Eddy to be mechanically possible. On melting, the body generally changes its volume, sometimes expanding, sometimes contracting. This may be explained by supposing that as the molecules are heated, their volumes diminish. The admissibility of this assumption has been proved by Lorentz and Sutherland. The change in volume on melting is then the resultant of the expansion due to the increased molecular motion and the contraction due to the shrinking of the molecules, and it may therefore be either positive or negative.

After melting, the temperature of the body continues to rise and the body generally expands until the boiling-point is reached; at that point the temperature again ceases to rise and the liquid becomes a vapor. We explain this by supposing that in consequence of the changes in velocity which go on among the molecules, there will arise an assemblage of molecules in a small region with velocities above the average; these will beat back the surrounding molecules and form a small bubble within which the molecules are in the gaseous state. Those molecules near the surface of this bubble which possess velocities above the average will pass through the liquid surface against the attractions of the molecules surrounding them and will increase the gas contained in the bubble, until its size becomes such that its buoyancy is able to overcome the viscosity of the liquid, so that it rises and sets free a number of molecules at the surface of the liquid in the gaseous state. The equality of