Abstract
The [S II] line ratio 6717/6731 observed in large galactic supernova remnants (SNR's), has a negative gradient as a function of galactocentric distance. This gradient may be at least partially due to a negative density gradient in the warm component of the interstellar medium. This hypothesis is verified by galactocentric distribution of maximum sizes of radio SNR's, where the largest objects are seen at greatest distances from the galactic center. A negative thermal pressure gradient is obtained when this density gradient and the temperature gradient obtained from radio observations of H II regions are combined. The shape of the density and pressure gradient is similar to the way in which molecular clouds and H166a are distributed. This similarity is probably due to the fact that at points interior to the solar galactocentric distance, the external pressure exceeds a critical pressure beyond which the conversion of diffuse into molecular clouds is possible.
