'Like nets or cobwebs': Kenelm Digby, Isaac Newton and the problem of rarefaction

This article aims to bring out the problematic nature of condensation and rarefaction for early modern natural philosophers by considering two historically significant attempts to deal with it, first by Sir Kenelm Digby in his Treatise on Body (1644), and subsequently by Isaac Newton, chiefly in manuscript works associated with the Principia (1687). It is argued that Digby tried to sidestep the problem of variation in density and rarity by making it a fundamental starting point for his physics. But he also brought out the difficulties of dealing with condensation and rarefaction within the mechanical philosophy, whether that philosophy was plenist or allowed for void space. The problems became exacerbated after experiments with the air-pump achieved extreme rarefactions. It is argued that these led Newton to first consider a retiform or net-like structure of matter, before adopting the radical innovation of supposing repelling forces operating at a distance between the particles of the rarefied bodies. Eventually, Newton came to believe that extreme rarity was inexplicable 'by any other means than a repulsive Power'.