The Collaborative International Dictionary of English v.0.48:

Rarefiable \Rar"e*fi`a*ble\ (r[a^]r"[-e]*f[imac]`[.a]*b'l), a.
   [Cf. F. rar['e]fiable.]
   Capable of being rarefied. --Boyle.
   [1913 Webster]