The following paper is a continuation of a former brief one, communicated to the Society, and printed in the Proceedings, on the change of electric resistance of iron due to change of temperature. In a note appended to Prof. Tait's paper on a “First Approximation to a Thermo-electric Diagram” (Trans. R.S.E., 1872–73), attention was drawn to the curious phenomenon observed by Gore, that at a temperature about dull red heat, iron wire undergoes sudden changes in length, and also to the further discovery by Prof. Barrett, that if the wire be cooling, a sudden reglow occurs simultaneously with these changes. These phenomena seemed to be connected with other known physical changes which take place in iron at this critical temperature, such as the loss of its magnetic properties, the remarkable bend of the iron line in the thermo-electric diagram, and the interesting alteration in the rate of change of electric resistance with respect to change of temperature, observable in iron at the same dull red heat. The following experiments were made mainly with the view of more thoroughly investigating this last peculiarity.