Mr. Fleeming Jenkin’s opinion only differs from mine in his ascribing the energy of sound acquired by a telephone with an iron diaphragm to the preponderance of sounds in the latter, whereas I consider it to be chiefly due to the increase of energy in the whole magnetic system produced by the reaction of the two magnetic parts on each other. If the two effects could be taken singly, it is probable that the sounds produced by each of them separately would be similar, since in magnetic effects the reaction and action are equal. But as they are combined, it becomes difficult to assign to each the share which belongs to it in the general effect observed. Besides, it is quite possible that the sounds of the diaphragm may appear to be stronger and more distinct because it is nearer to the ear than the magnet, and because the effects of magnetisation and demagnetisation are then more easily produced in consequence of the mass of the magnetic body being smaller. Mr. Fleeming Jenkin goes on to say that the question of the displacement of surface in the diaphragm and magnet is very complex, but that he thinks it impossible to deny the existence of such displacement, since the air which acts as the vehicle of sound between the ear and the source of sound is placed in vibration; yet this displacement maybe effected quite otherwise than by flexion. Suppose that the magnetic molecules of these bodies are drawn together by magnetisation, which tends to diminish the intermolecular space which separates them, the points of surface of the substance corresponding to these intervals will be elevated in a manner equivalent to a displacement of surface, and the effect of this will be the same as a flexion movement. At the moment of demagnetisation a depression instead of an elevation will take place, and the vibratory movements will thus be produced without any electro-magnetic attraction, and it is precisely these vibrations which Mr. Fleeming Jenkin terms molecular vibrations. He evidently does not mean that such attractions cannot take place: they may react, together with the molecular vibrations, when the electric force is capable of producing them. He adds that the reproduction of sounds by a condenser, by simple coils, and by a carbon microphone, has convinced him that the action just analysed requires generalisation.