The Herschel-guaranteed time key programme PRobing InterStellar Molecules with

Absorption line Studies (PRISMAS)1 is providing a survey of the interstellar hydrides

containing the elements C, O, N, F and Cl. As the building blocks of interstellar molecules,

hydrides provide key information on their formation pathways. They can also be used

as tracers of important physical and chemical properties of the interstellar gas that

are difficult to measure otherwise. This paper presents an analysis of two sight-lines

investigated by the PRISMAS project, towards the star-forming regions W49N and W51.

By combining the information extracted from the detected spectral lines, we present an

analysis of the physical properties of the diffuse interstellar gas, including the electron

abundance, the fraction of gas in molecular form, and constraints on the cosmic ray

ionization rate and the gas density.