Professor Lighthill's paper has discussed the dissociative behaviour of air at the high temperatures produced when it is compressed in a shock or heated by friction in a boundary layer under hypersonic flight conditions. Just as molecules may dissociate into atoms, so may these molecules and atoms split further into charged ions and electrons. The equilibrium state of this ionisation process is amenable to the same thermodynamical or statistical-mechanical treatment as in the case of dissociation, the Law of Mass Action in this context being referred to as the Saha equation. Consider the simplest case where a monatomic gas is singly ionised, a fraction α of the atoms having lost one electron, so becoming singly charged positive ions.