Copy letter, from Sir John Herschel to John Tyndall, dated at Collingwood Copy of a letter, with diagrams. Thanks for and comments on papers on scientific history and molecular physics, particularly on a magnetic experiment and on dynamical efficiency of coal relative to motion theory of light and heat. Comments on nature of heat, 'vis viva.' Deplores laxity with which physicists use and confuse terms 'force,' 'motion,' and 'energy.' 18 July 1864