Recently developed instruments such as the Taurus Tunable Filter and WHAM should be able to detect some or all of the optical emission lines H$α$, [OI] $λ$6300, [SII] $λ$6717, [NI] $λ$ 5200 and [NII] $λ$6584 from warm interstellar clouds such as those observed by Spitzer $&$ Fitzpatrick (1993) (SF) along the line of sight to the halo star HD93521. The strengths of these lines should resolve the debate as to whether the free electrons, which SF held responsible for the observed excitation of CII in the clouds, are located mainly in the skins of the clouds or in their interiors. If the free electrons are indeed mainly located in the cloud interiors, then the substantial electron density derived by SF, and its constancy from cloud to cloud for the slow-moving clouds, when combined with their opacity to Lyman continuum radiation, lend strong support to the decaying neutrino theory for the ionisation of the interstellar medium (Sciama 1990, 1993 a, b, 1997). If the [OI] and [NI] lines are relatively strong but the [NII] line is weak, then this would lend further, decisive, support to this theory, since decay photons are unable to ionise N, although its ionisation potential is only 0.9 eV greater than that of H.

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