1. [v]  weaken the consistency of (a chemical substance)
2. [v]  make more subtle or refined
3. [v]  lessen the density or solidity (of a substance); "The bones are rarefied"

RAREFY is a 6 letter word that starts with R.

1. \Rar"e*fy\, v. t. [imp. & p. p. {Rarefied}; p. pr. & vb.
   n. {Rarefying}.] [F. rar['e]fier; L. rarus rare + -ficare (in
   comp.) to make; cf. L. rarefacere. See {-fy}.]
   To make rare, thin, porous, or less dense; to expand or
   enlarge without adding any new portion of matter to; --
   opposed to condense.
   
   

2. \Rar"e*fy\, v. i.
   To become less dense; to become thin and porous. ``Earth
   rarefies to dew.'' --Dryden.