1. Limit of validity of formulas

The theory allows us to treat these transport properties within the same framework of reasoning for a very dilute gas, i.e. one in which pressure has no appreciable influence on viscosity and conductivity. It is assumed that collisions only occur between two molecules (no simultaneous collisions of three or more molecules). Contrary to what one might think, this clause is not very stringent; after re-examining a study that was over-optimistic in its conclusions, we are led to say that the following applies, to within better than 1%, to gases in a pressure-temperature range delimited by :