Whereas most seventeenth-century atomists accounted for rarefaction in terms of the intrusion of tiny void spaces within the particles of a body, Descartes devised an explanation that was in agreement with his plenist physics. Some letters to Mersenne, written between 1629 and 1630, bear witness to this attempt. In the first of them, dated October 8, 1629, Descartes praises the explanation of rarefaction provided by a certain doctor, most probably Sebastian Basso, although, he adds, “I do not explain the ether as he does” (AT I 25). While Basso believed that rarefaction was caused by a separate ethereal substance entering the pores of material bodies, Descartes argued in a subsequent letter that the corpuscles that cause rarefaction “are of the same substance of visible and tangible bodies.” These corpuscles must not be regarded as atoms but as “an extremely fluid and subtle substance” (AT I 140, CSM I 22). Contrary to Basso's ether, Descartes’ subtle matter is transmutable into the other elements, from which it differs only for the size and shape of its particles (Ariew 2011).

In another letter to Mersenne, dated February 25, 1630, Descartes explains rarefaction and condensation by using the example of a sponge that expands upon absorbing water and contracts upon being squeezed (AT I 119). This analogy reappears in Descartes’ works. In chapter 5 of The World, one reads that material bodies are, properly speaking, composed only of the third element, although they also contain particles of the other two elements. These bodies can be compared to a sponge, the pores of which “are always full of air or water or some similar fluid,” which, however, do not “enter into its composition” (AT X 31, G 21).

The analogy of a sponge is invoked in the Principles of Philosophy II.6 to explain that rarefaction involves a change of shape of a body's particles but not an increase in its extension: