Letters written to Mackintosh chiefly on scientific subjects, from his colleagues including Thomas Sterry Hunt (1826-1892), 37; William Earl Hidden (1853-1918), 50; Thomas Egleston (1832-1900), founder of the School of Mines, Columbia University, 10; R.S. Penniman, 26; and Pierre Eugene Marcelin Berthelot (1827-1907), eminent French chemist, 1. Also, miscellaneous documents relating to Mackintosh and about fifteen printed articles and brochures.