The Collaborative International Dictionary of English v.0.48:

Evanesce \Ev`a*nesce"\ ([e^]v`[.a]*n[e^]s"), v. i. [imp. & p. p.
   Evanesced ([e^]v`[.a]*n[e^]st"); p. pr. & vb. n.
   Evanescing. ([e^]v`[.a]*n[e^]s"s[i^]ng).] [L. evanescere; e
   out + vanescere to vanish, fr. vanus empty, vain. See Vain,
   and cf. Evanish.]
   To vanish away; to become dissipated and disappear, like
   vapor.
   [1913 Webster]

         I believe him to have evanesced or evaporated. --De
                                                  Quincey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

evanesce
    v 1: disappear gradually; "The pain eventually passed off" [syn:
         evanesce, fade, blow over, pass off, fleet,
         pass]