Except in particular situations, the behaviour of ordinary gases is adiabatic. In fact, due to the low values of the viscosity coefficients and thermal conductivity there is no thermal contact with the surrounding medium and the dissipation of energy by viscosity is negligible. In general, viscosity and thermal conduction are also of little importance in interstellar gas. Consider the role of thermal conduction in a molecular cloud of size \( 1\ \mathrm{pc} \), with temperature \( T = 10\ \mathrm{K} \) and density \( n = 100\ \mathrm{cm} ^{- 3} \).