London, The Royal Society, 1868. Large 4to (30.3 x 23.1 cm). 15 pp.; one text illustration [numbered 643-657 (658 blank)]. Contemporary marbled wrappers.

Important paper by the greatest 19th-century theoretical physicist, James Clerk Maxwell (1831-1875). The illustration is of a specially built apparatus needed to collect raw data. This is “his fifth major paper” (DSB), in which Maxwell condensed and simplified his description of the electromagnetic field. It ends with the conclusion that “the number of electrostatic units in one electromagnetic unit of electricity is numerically equal to the velocity of light” (p. 657). A very good, clean copy. DSB 9, p. 212.