Correspondence and papers, including laboratory and other notebooks; notes of lectures attended; manuscripts of his published works; notes and papers on astronomy, color and optics, electricity and magnetism, geometry, mechanics and dynamics, molecular physics and gases, the properties of matter and general physics. The most extensive and interesting portion of the correspondence (ca. 160 items, 1859-1879) is with Peter Tait. Maxwell and Tait discuss the problems then current in the foundation of mathematical physics -- quaternions, spherical harmonics, and thermodynamic equations, while also including algebraic conundrums and puns and jokes in mathematical symbolism. Correspondents include: Michael Faraday, Lord Kelvin, Charles Tomlinson, and John Tyndall.