The derivation of the spatial distribution of cosmic rays from an analysis of cosmic gamma rays requires a knowledge of the distribution of gas in the inner Galaxy most notably that of molecular hydrogen. There has been considerable argument about the amount of this gas, an argument manifested by disagreement over the magnitude of the conversion factor, X, used in converting the strength of the 12CO line to the column density of molecular hydrogen. A promising technique has been to study the far infrared radiation from dust associated with gas but here, too, there have been differences. The authors analyse the situation in detail and conclude that virtually all the differences can be understood and that the best estimate in the inner Galaxy is XMS approximately 1, in units of 1020 H2 molecules cm-2, the CO data used being those from the Massachusetts-Stony Brook survey of Sanders et al. (1986).