Autograph letter signed : [London], to an unidentified recipient, [1865] June 23.

Year of writing suggested by internal evidence; 1865 was only year when it can be confirmed that Tyndall had lectured at Cambridge, on 16 May. Cf. On radiation. The "Rede" lecture, delivered in the Senate-house before the University of Cambridge on Tuesday, May 16, 1865. By John Tyndall. London : Longman, Green, Longman, Roberts & Green, 1865.