1. Gases concerned

A gas is defined as "any body in an expandable and compressible fluid state (gaseous state) under conditions of normal temperature and pressure" [Robert, 1990].

A permanent gas is "a gas that cannot be liquefied by a simple increase in pressure" [Larousse, 1972], meaning at ordinary temperature, i.e. without the application of cold. In more scientific language, we would say: any gas whose critical temperature is below ambient. However, it is important to specify where the ambient temperature lies. Examination of the network of gas vapour pressure curves (figure 1 ) shows that, with the exception of ozone (a molecule that is difficult to encounter in its pure state due to its instability), no gas has its critical point in the temperature zone between...