incoercible

 Not to be coerced; incapable of being compelled or forced.  [1913 Webster]

 Not capable of being reduced to the form of a liquid by pressure; -- said of any gas above its critical temperature.  [1913 Webster]

 That can note be confined in, or excluded from, vessels, like ordinary fluids, gases, etc.; -- said of the imponderable fluids, heat, light, electricity, etc.  [1913 Webster]