"[I]t was formerly assumed, in determining the volumes of the unit of weight of saturated vapour at different temperatures, that vapour even at... maximum density... obeys Mariotte's and GayLussac's laws. ...I have ...shown in my first memoir... the volumes in question can be calculated... under the assumption, that a permanent gas when it expands at a constant temperature only absorbs so much heat as is consumed in the external work thereby performed, and that these calculations lead to values which, at least at high temperatures, differ considerably from Mariotte's and Gay-Lussac's laws."