Ebullition, or boiling, signifies the rapid formation of vapor bubbles in the mass of a liquid. When a flask containing water is placed over the flame of a lamp, the absorbed air that is generally to be found in water is driven off in minute bubbles that rise and escape without noise. As the temperature of the water is raised, the liquid molecules in contact with the bottom of the flask become so hot that the heat is able to overcome the cohesion between the molecules, the pressure of the overlying water, and the pressure of the atmosphere above the water. Then the water boils. (a.) When the first bubbles of steam are formed at the bottom of the water, they rise through the water, condense in the cooler layers above, and disappear before reaching the surface. The formation and condensation of these bubbles produce the peculiar sound known as singing or simmering, the well-known herald of ebullition. Finally, the water becomes heated throughout, the bubbles increase in number, grow larger as they ascend, burst at the surface, and disappear in the atmosphere. The whole liquid mass is agitated with considerable vehemence, there is a characteristic noisy accompaniment, the quantity of water in the flask diminishes with every bubble, and finally it all disappears as steam. The water has "boiled away."