rarefy

verb

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[…]”

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[…]”

   “As water rarefies, it evaporates, forming visible steam, and this may be a step toward the final category of reality, fire. Anyone who has held a hand over a boiling kettle may feel the truth of this.[…]”

   “Given the high quantity of water molecules in the human body it is worthy of note at this point that moisture rarefies when heated.[…]”