By the kindness of Dr Russell, of Bartholomew's Hospital, I have been enabled to experiment upon a ribbon of pure iron prepared by the late Dr Matthiessen. I reserve details until I can obtain the history and mode of preparation of the specimen examined, but I may state now that, when formed into thermo-electric circuits with various alloys of Platinum and Iridium (Proceedings, 1871–2, p. 773) it gives results, as to the position of neutral points, not differing more from those given under the same circumstances by various iron wires of commerce, than the latter do among themselves. Thus it appears that, in the thermo-electric diagram, the line even for pure iron is sinuous; and that the specific heat of electricity in it changes sign somewhere about a low red heat.