That's a classic version of that song by that first line up, very rare and Blackmore not yet having perfected his later "man in black with solemn introverted gaze" stage persona. He's still all smiles.
That first line up has its charm, but you can't deny that Evan's vocals were very sixties/lounge style (though Ian Gillan is on record for liking Rod's vocals) and that he really did not have the pipes to cut through hard charging guitar riffs (which is where they wanted to move their music to). Gillan's first recording with DP was a cover of a Jesus Christ Superstar type song (though it preceded the musical quite a bit)
quite removed from the In Rock sessions that were also taking place at the time, but you can already hear why he was more suited for where DP wanted to go, just listen at 1.28 and 2.50 for his trademark silver-throated screamings. In the late eighties, upon the reunion of the Mk II line up with Gillan, they even revisited Hush in a slightly gutsier version:
http://www.youtube.com/watch?v=Ilz5SxrHA3E&feature=relatedUwe
