YOUR issue of June 30, containing a letter from M. Henry Bourget under the above heading, has only just come into my hands. In 1889, when working at the Cavendish Laboratory, Cambridge, I was interested in the phenomenon described by M. Bourget, and made a few preliminary experiments in connection therewith, but, owing to other work, I was unable to proceed very far with the investigation, and probably shall not have the opportunity of doing anything further. My notes, taken at the time, show that the following experiments were made and results obtained:—A large well-used soldering bolt had a hole drilled nearly through the iron shank at the end remote from the copper portion; into this hole, and surrounded with mercury, was placed the bulb of a thermometer with small thermal capacity; a bunsen flame was then placed beneath the copper portion, and the heating continued until the thermometer indicated a steady temperature. The flame was then removed, and a vessel containing water brought up over the hot end, upon which the thermometer showed a rise of 1˙5° C. There was a considerable film of copper and iron oxides, &c., at the junction of the iron and copper.