Place in the receiver a tumbler of cold water and work the pump as before. In a few minutes, as soon as the air is sufficiently exhausted, the water will apparently boil. Yet you know the water does not boil in a kettle unless heated to 212 degrees. This phenomenon is thus explained: The vacuum causes the air-bubbles contained in the water to escape. They easily do so, because there is scarcely any reserve on the surface of the liquid (see fig.).