One little caveat, TEd. When you compress gas, you do work on it, and that takes the form of heat.A simple demonstration of this is to take a bicycle pump, put your finger over the place where the compressed air comes out, and push the plunger in -like a porcupine making love- very cautiously. If you press the finger over the opening too tightly, and move the plunger too powerfully, you can get a painful blister.