When water or any other liquid is heated in the open air, its temperature rises. After a time bubbles of vapor are formed and reach the surface, and at this stage boiling or ebullition has begun. The heat converts the water from the liquid state into the state of gas or vapor, which rises in bubbles and passes off into the air as steam. The temperature at which boiling begins is called the boiling-point. Different liquids have different boiling points, - that of water being 2120 F.; of alcohol, 173.120 ; of mercury, 6620 ; and of acetic acid, 243.140. These are the temperatures needed at sea-level. At higher points, where the pressure of the atmosphere is less, the boiling point is reached at lower temperatures.