In this chapter we shall be first concerned with theories designed to explain the properties of bodies, starting from first principles. These atomic theories fall into two groups; those such as Mosotti’s involving more than one kind of fundamental particle, and the theory of the vortex atom, which was a compromise between an atomic theory and the view that matter is a continuum. These theories were developed by physicists and although their authors sought in a general way to account for the chemical properties of bodies, chemists seem to have paid little attention to them. Such theories solved no chemical problems, and therefore belong on the physical side of the divide between the two sciences; the object of all their authors was to postulate particles as simple as possible, which would nevertheless explain a wide range of properties. Also in the realm of physics was the dynamical or kinetic theory of matter, the object of which was to account in detail for the properties of gases—and in general for those of solids and liquids—on the assumption that they are composed of particles in rapid motion. Those responsible for this theory tended, like the chemists, to be reluctant to commit themselves to an atomic theory, and rested content with having made it very probable that matter was made up of molecules or particles. The theory is of great importance in our story because its conclusions were in accord with those deduced from the facts of chemistry; and 61we shall therefore pass on to it after discussing the vortex atom.