rarefaction

noun

1. (countable, uncountable)A reduction in the density of a material, especially that of a fluid.

   “[T]his ſeal'd vial, being broken under water, ſuck'd in a conſiderable quantity of it, whether, becauſe of ſome little rarefaction of the Air included in the ſealing, or becauſe of the infrigidation of that Air by the ſnow, or for both theſe Reaſons, or any other, I ſhall not Now diſpute.”