A. It is an invisible elastic gas generated from water by the application of heat.
Q. What are its properties which make it so valuable to us?
A. 1.—The ease with which we can condense it. 2.—Its great expansive power. 3.—The small space it occupies when condensed.
Q. Why do you condense the steam?
A. To form a vacuum and so destroy the back pressure that would otherwise be on the piston and thus get more useful work out of the steam.
Q. What is vacuum?
A. A space void of all pressure.
Q. How do you maintain a vacuum?
A. By the steam used being constantly condensed by the cold water or cold tubes, and the air pump as constantly clearing the condenser out.
Q. Why does condensing the used steam form a vacuum?