If ice be heated above 32° Fahrenheit, its molecules lose their
cohesion, and move freely round one another--the ice is turned into
water. Heat water above 212° Fahrenheit, and the molecules exhibit a
violent mutual repulsion, and, like dormant bees revived by spring
sunshine, separate and dart to and fro. If confined in an air-tight
vessel, the molecules have their flights curtailed, and beat more and
more violently against their prison walls, so that every square inch of
the vessel is subjected to a rising pressure.