Of two letters written by William Thomson (Lord Kelvin) to the Genevese physicist Auguste de la Rive (1801-1873) preserved in the public University Library of Geneva, one is of distinct interest. This letter (M.S. 2319), written on 17 December 1856, throws sidelights on the discovery of the 'Thomson effect' (originally described in his paper to the Royal Society of Edinburgh in 1851) and on the state of his thought about the nature of the mobile element involved in electrical conduction.