Voltaic Decomposition of Solutions. 163 



test the nature of the action would be to employ some solution 

 which would yield a matter capable, by deposition in the sub- 

 stance of the membrane, of increasing the obstacle, and would 

 likewise contain another matter which might be the subject of 

 a secondary agency, if it really was hydrogen which experi- 

 enced the difficulty in uniting with oxygen coming from the 

 other side of the septum. The salt which appeared to me to 

 offer these conditions was iodate of magnesia. If a solution 

 of it were substituted for sulphate of copper, then if magne- 

 sium were to be carried by direct agency towards the mem- 

 brane, we should of course simply expect that, as in the case 

 of sulphate of magnesia, a deposit of magnesia would take 

 place on the diaphragm, the affinity of magnesium for oxygen 

 being too strong to allow any of it to escape combination with 

 the oxygen coming in the opposite direction. On the other 

 hand, if water was the subject of direct decomposition, and if 

 from the contemporaneous decomposition of iodate of magnesia 

 into its constituent acid and alkaline earth and precipitation 

 of magnesia on the septum, an obstacle should be offered to 

 the whole of the hydrogen uniting with the whole of the oxygen 

 passing in the opposite direction, we should expect that the 

 portion of hydrogen escaping union would reduce iodic acid 

 at the septum, as it does when it reaches the pole immersed 

 in a solution of that acid or of one of its salts. 



The arrangement I employed for the experiment was very 

 simple. A circular piece of bladder was cut rather larger than 

 a watch-glass, and a segment of this circle, somewhat larger 

 than the half, was moistened and made to adhere to a half of 

 the hollow surface of the watch-glass, a portion of it being at 

 the same time made to stand perpendicularly in the direction 

 of the diameter of the circle, so as to form a septum across it, 

 the remainder of the edge being bent over the outer edge of 

 the watch-glass so as to adhere to it, and the whole being then 

 allowed to dry. In this way, by pouring small quantities of 

 two liquids on each side of the central diaphragm, they could 

 be kept separate for a sufficient time to try voltaic action on 

 them ; it being found, however, that some fluids answered 

 better to be placed on the bladder-side and others on the other 

 division, so as to prevent subsequent mixing. When a dilute 

 solution of potash was placed on the bladder-side of the sep- 

 tum and a solution of sulphate of copper on the other, and 

 the former made negative and the latter positive by platinum 

 wires from thirty-six pairs of four-inch plates, the side of the 

 diaphragm towards the sulphate of copper was found in a 

 quarter of an hour or twenty minutes to exhibit metallic cop- 

 per lying on a dense layer of blue oxide of copper, 



M 2 



