Mawson, C.O.S., ed. (1870–1938). Roget’s International Thesaurus. 1922.

Class III. Words Relating to Matter
Section II. Inorganic Matter
1. Solid Matter

322. Rarity.

   NOUN:RARITY; tenuity; absence of solidity [See Density]; subtility; subtilty; sponginess, compressibility.
  rarefaction, rarefication [rare], expansion, dilatation, inflation, subtilization.
  ether (gas) [See Gaseity].
   VERB:RAREFY, expand, dilate, subtilize [rare], attenuate, thin.
   ADJECTIVE:RARE, subtile [now rare], subtle, thin, fine, tenuous, compressible, flimsy, slight; light [See Levity]; cavernous, porous, spongy (hollow) [See Concavity].
  rarefied &c. v.; unsubstantial; uncompact, uncompressed; rarefiable, rarefactive, rarefactional.
  1. Melted into air, thin air.—Tempest
  2. I pass through the pores of the ocean and shores.—Shelley, Cloud