Newton's 'De Aere et Aethere' and the introduction of interparticulate forces into his physics.

As well as the mathematically-supported celestial mechanics that Newton developed in his Principia, Newton also proposed a more speculative natural philosophy of interparticulate forces of attraction and repulsion. Although this speculative philosophy was not made public before the 'Queries' which Newton appended to the Opticks, it originated far earlier in Newton's career. This article makes the case that Newton's short, unfinished manuscript, entitled 'De Aere et Aethere', should be seen as an important landmark in Newton's intellectual development, being the first work in which Newton assumed there are repulsive forces operating at a distance between the particles of bodies. The article offers an account of how Newton came to write 'De Aere et Aethere' and why. It also outlines its relationship to the 'Conclusio', with which Newton briefly intended to finish the Principia, and to the 'Queries' in the Opticks. The date of the manuscript is disputed, and the article also aims to settle this dispute. Claims that the 'De Aere et Aethere' must have been written before the 'Hypothesis... of Light' of 1675 are dismissed, and it is suggested, following R. S. Westfall, that it was written after a well-known letter Newton wrote to Boyle early in 1679.