The paper began by pointing out the very limited class of bodies to which observations of the kind alluded to have been extended. The author was led to a careful examination of the existing determinations of the conducting powers of the metals for heat, by some collateral trains of experiment in which he was occupied two years ago. He points out the degree of confidence which may be placed in the arrangement of conductors given by different authors; for we appear to be far from reaching a correct estimate of their numerical values. In viewing those of M. Despretz as the best, he remarks, that the position of platinum, which is certainly erroneous, shews how imperfectly we can depend upon experiments on this point, made with even more than usual care. Platinum is placed, by this writer, almost at the top of the list, between gold and silver, whilst the commonest experiments serve to shew that it is really a very imperfect conductor.