Ebullition, or Boiling, signifies the free passage of the particles of a liquid upwards from its free surface level into the space above. It can only occur when the temperature of the liquid is such that the particles exert sufficient pressure to overcome the superincumbent gaseous pressure, and so pass freely from one side to the other of the surface level. For every given external pressure on unit area of the liquid, there is a corresponding temperature of ebullition or boiling-point. But it must be understood that a certain amount of diffusion of the water-particles may take place even when their pressure is insufficient to allow perfectly free passage from the liquid to the gaseous condition. This diffusion is called evaporation. (q.v.); it is identical with ebullition only when the vapour pressure is equal to that special pressure of the liquid that corresponds to the given temperature. Thus evaporation of a liquid in contact only with its own vapour is really ebullition, and the vapour is then said to be at dew-point (q.v.), or to be saturated.