"[T]he entire quantity of heat, Q, absorbed by the gas during a change of volume and temperature may be decomposed into two portions. One of these, U, which comprises the sensible heat and the heat necessary for interior work, if... present... determined by the state of the gas at the beginning and at the end of the alteration; while the other portion... the heat expended on exterior work, depends, not only upon the state of the gas at these two limits but also upon the manner in which the alterations have been effected..."