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rarefication

rarefiˈcation rare.
  Also 7–8 rari-.
  [Noun of action, after L. types, from rarefy v.]
  = rarefaction.

1616 in Bullokar Eng. Expos. 1727 Bradley Fam. Dict. s.v. Bee hive, Its Whiteness is increased by this Rarification. 1794 Sullivan View Nat. I. 209 It carries the point of greater rarefication on the other side of the equator. 1893 Sir J. C. Browne in Times 3 Oct. 9/5 Rarefication in quality of two orders of impressions.

Oxford English Dictionary

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