Correspondence and notebooks. Two series of correspondence include in-coming letters (1875-1911) of a scientific and professional nature primarily on research in acoustics, fluids and electromagnetism from over 350 scientists, and a second series (Argon correspondence, 1887-1904) consisting chiefly of Rayleigh - Sir James Ramsey letters and Rayleigh - Dewar letters focusing on Rayleigh's discovery of argon; included are some calculations and data readings as well as congratulatory letters following his announcement of the discovery. Notebooks (1864-1919) range from student notes Rayleigh took on topics including hydrodynamics, probability, and mathematics and from William Stoke's lectures on optics, as well as notes which he prepared for lectures on thermodynamics while Cavendish was Professor at Cambridge University and lectures elsewhere on induced currents and magnetism. Experimental notes from his research are on optics, acoustics, magnetism and electricity. Correspondents include: J. Aitken, William Crookes, James Dewar, A. J. Ellis, J. D. Everett, R. E. Froude, Francis Galton, A. R. Hunt, Horace Lamb, Joseph Larmor, Oliver Joseph Lodge, Augustus Edward Hough Love, Arnulph Mallock, Eleuthre Mascart, Alfred Marshall Mayer, Albert A. Michelson, William Ramsey, Arthur Schuster, Peter G. Tait, John Tyndall, and Robert W. Wood.