In accordance with these effects, it is intelligible that if two tubes T T, pointed at the end, and containing mercury, are plunged into two vessels V V (fig. 34) containing acidulated water and mercury, and metallic wires, P P, Q Q, are used, first to connect the columns of mercury in the tubes, and secondly the layers of mercury at the bottom of the two vessels, the tubes being a little removed from the surface of the mercury in the vessels, we shall then have a metallic circuit, completed by two electrolytes, one of which will be subjected to the mechanical or electrical effects produced in the other. If two vibratory plates B B are placed above the tubes, and one of these is caused to vibrate, the other will reproduce these vibrations, influenced by the vibratory movements communicated by the corresponding column of mercury. The vibrations themselves will be in connection with the electrical discharges resulting from the movements of the column of mercury in the first tube, which are mechanically produced. If an electric generator is introduced into the circuit, the effect which we have just analysed will be caused by modifications in the potential of this generator, in consequence of electro-capillary effects. But if no generator is employed, the action will result from electric currents determined by the electro-capillary attraction itself. In the latter case, however, the instrument must be more delicately made, in order to obtain more sensitive electric reaction, and M. A. Bréguet describes his instrument as follows. ‘The instrument consists of a tube of thin glass, a few centimètres in length, containing alternate drops of mercury and acidulated water, so as to constitute so many electro-capillary elements, connected in tension. The two ends of the tube are fused together, yet so as to allow a platinum wire to touch the nearest drop of mercury on each side. A small circle of thin deal is fixed at right angles to the tube by its centre, thus providing a surface of some extent, which can be applied to the ear when the instrument is a receiver, and to make the tube more mobile under the influence of the voice when the instrument is a sender. The following are the advantages offered by instruments of this construction:-- ‘1. They do not involve the use of a battery. ‘2. The disturbing influence of the resistance of a long line is almost destroyed in these instruments, although it is still appreciable in the Bell telephone. ‘3. Two mercury telephones, coupled together as we described above, are absolutely correlative, in this sense, that even different positions in the equilibrium of the mercury in one of them produce different positions of equilibrium in the opposite instrument. It is therefore possible to reproduce at a distance, without a battery, not merely faithful indications of oscillatory movements, which is done by the Bell telephone, but also the exact image of the most general movements.’