1. The HI envelopes were discovered in galaxies of all types. The optical images of galaxies are always seen in the regions where the HI surface density NHI is greater than some critical value N1 ≅ 5·1020 cm−2. The molecular gas is embedded in those parts of galaxies where NHI ≥ N2 − 2·1021 cm−2. As follows from kinetics of molecule formation the molecular hydrogen forms at column densities greater than N1 and CO formation requires N ≥ N2 (Arshutkin and Kolesnik (1981,Astrofizika. 17.359; 1984, ibid.21.147)). These results explain why the observations of external galaxies emphasize the column densities N1 and N2 as critical ones.