rarefying

Verb

rarefy (third-person singular simple present rarefies, present participle rarefying, simple past and past participle rarefied)

1. (transitive) To make rare, thin, porous, or less dense.
   • By means of this gauge, Mr. Smeaton judged that his machine was incomparably better than any former one, as it seemed to rarefy the air in the receiver 1000, or even 2000 times, while the best of these only rarefied it about 140[…]1815, A Philosophical and Mathematical Dictionary: Containing an Explanation of the Terms, and an Account ... by Charles Hutton ... vol. 1[-2], page 56
2. (intransitive) To become rare, thin or less dense.
   • For if The^([sic]) water rarefies and then Condenses, all that great mass of water of the Lake of the Ilinois rises in its vast basin when it rarefies, and falls when it condenses. And, as water always rises as much as it falls, it would[…]1916, Wisconsin Geological and Natural History Survey, Bulletin, page 293