Roget's Thesaurus: Entry 322 (Rarity)

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#322. Rarity. -- N. rarity, tenuity; absence of solidity &c. 321; subtility[obs]; subtilty[obs], subtlety; sponginess, compressibility.

rarefaction, expansion, dilatation, inflation, subtilization[obs].

vaporization, evaporation, diffusion, gassification[obs].

ether &c. (gas) 334.

V. rarefy, expand, dilate, subtilize[obs].

Adj. rare, subtile, thin, fine, tenuous, compressible, flimsy, slight; light &c. 320; cavernous, spongy &c. (hollow) 252.

rarefied &c. v.;; unsubstantial; uncompact[obs], incompressed[obs]; rarefiable[obs].