RAREFACTION • E-FREE TRANSLATION.COM • Definition of Rarefaction

... extension, in that which constitutes the object of geometry, in that magnitude which is divisible, figurable, and movable. This thesis (corpus extensio sive spatium) is next defended by Descartes against several objections. In reply to the objection drawn from the condensation and rarefaction of bodies, he urges that the apparent increase or decrease in extension is, in fact, a mere change of figure; that the rarefaction of a body depends on the increase in size of the intervals between its parts, and the entrance into them of foreign bodies, just as a sponge swells up when ...

— History Of Modern Philosophy - From Nicolas of Cusa to the Present Time • Richard Falckenberg