Until a few years ago it was the general opinion among metallurgists that the presence of manganese in steel exceeding the proportion of 1 per cent, is prejudicial to the value of the steel, inasmuch as a higher percentage of manganese has the effect of lowering markedly its tensile strength and toughness. But in 1884, Messrs Hadfield & Company, of the Hecla Steel Works, Sheffield, exhibited, at a meeting of the Institute of Mechanical Engineers, a number of samples of steel containing upwards of 10 to 15 per cent, of manganese, and submitted the results of experiments, which showed that the samples were, in point of tensile strength and hardness, in no way inferior to steel. Again, in 1888, Mr R. A. Hadfield read to the Institute a paper on the subject, giving the details of a large number of tests, which brought to light some interesting mechanical properties of alloys of manganese and iron. Since its introduction, these alloys (and particularly that containing 10 to 15 per cent, of manganese, known as “manganese-steel”) have been studied by several physicists, and further peculiarities have been found. It appeared desirable that the thermal conductivity of so peculiar a substance should be investigated. The present paper is an account of experiments made in the Physical Laboratory, Edinburgh University, with a view to the determination of its thermal conductivity. In the reduction of such experiments a knowledge of the specific heat is necessary, hence there is also given an account of experiments whereby the specific heat was determined.