In gas dynamics we are interested in the local relations between pressure, density and temperature, rather than considering a fixed quantity of gas. By considering the density ρ ρ = M / V as the inverse of the volume for a unit mass, we can take ρ ρ = 1 / V in these relations. Since for constant entropy, S , we have P ∝ ∝ ρ ρ γ γ , or ln ⁡ ⁡ P = γ γ ln ⁡ ⁡ ρ ρ + c o n s t a n t , it follows that