"[A] fact proved by Rankine and myself... when a quantity of vapour, at its maximum density... enclosed by a surface impenetrable to heat, expands and thereby displaces... e, g. a , with its full force of expansion, a part of the vapour must undergo condensation; whereas in most works on the steam-engine, amongst others in the excellent work of De Pambour, Watt's theorem, that under these circumstances the vapour remains... at... maximum density, is assumed... fundamental..."