A. By bringing the body back from a state of gas to a liquid, or from a liquid to a solid.
Q. If the power is in coal, why should we use steam?
A. Because steam has some properties which make it an invaluable agent for applying the energy of the heat to the engine.
Q. What is steam?
A. It is an invisible elastic fluid generated from water by the application of heat.
Q. What are its properties which make it so valuable to us?
A. First. The ease with which we can condense it.
Second. The small space which it occupies when condensed.
Third. Its great expansive power.