In his book (see references [3] and [4]) Boltzmann wrote equations using V / M (specific volume) in place of V N A / N (molar volume) used here, Gibbs did as well, so do most engineers. Also the property, V / N the reciprocal of number density, is used by physicists, but there is no essential difference between equations written with any of these properties. Equations of state written using molar volume contain R , those using specific volume contain R / m ¯ ¯ (the substance specific m ¯ ¯ is the molar mass), and those written with number density contain k .