  In this paper we use a number of observations to construct an integrated
picture of the ionisation in the interiors of quiescent warm opaque
interstellar clouds and in the intercloud medium (ICM) outside dense HII
regions and hot dilute bubbles. Our main conclusion is that within $\sim$ 1kpc
of the sun the ionisation rate of hydrogen per unit volume in both the
interiors of such clouds and in the ICM is independent of the local density of
neutral hydrogen, and varies with position by less than $\sim$ 20 per cent.
These conclusions strongly favour the decaying neutrino hypothesis for the
ionisation of the interstellar medium in these regions.
  Our analysis is based on a variety of observations, of which the most
remarkable is the discovery by Spitzer and Fitzpatrick (1993) that, in the four
slowly moving clouds along the line of sight to the halo star HD93521, the
column densities of both SII and CII$^*$, which individually range over a
factor $\sim$4, are proportional to the column density of HI to within $\sim$20
per cent. This proportionality is used to show that the free electrons exciting
the CII to CII$^*$ are located mainly in the interiors of the clouds, rather
than in their skins, despite the large opacity of the clouds to Lyman continuum
radiation. The same conclusion also follows more unambiguously from the low
value of the H$\alpha$ flux in this direction which was found by Reynolds
(1996) in unpublished observations.
  These results are then used, in conjunction with observations of three pulsar
parallaxes and dispersion measures, and with data on HeI, NII and OI line
emissions, to constrain the ionisation of H, He, N and O and the flux of Lyman
continuum photons from O stars in the ICM.
