i;O THE SCIENTIFIC PAPERS OF 



may be eliminated by combining two similar voltameters in one 

 instrument, which I propose calling a " differential voltameter," 

 and which is represented in the drawing, Fig. 9, Plate 13. 



DIFFERENTIAL VOLTAMETER. It consists of two similar narrow 

 glass tubes, A and B, of about 2*5 millimetres in diameter, fixed 

 vertically to a wooden frame, F, with a scale behind them divided 

 into millimetres or other divisions. The lower ends of these tubes 

 are enlarged to about 6 millimetres in diameter, and each of them 

 is fitted with a wooden stopper saturated with paraffin and pierced 

 by two platinum wires, the tapered ends of which reach about 25 

 millimetres above the level of the stopper. These form voltametric 

 electrodes. 



From the enlarged portion of each of the two voltameter tubes 

 a branch tube emanates, connected, by means of an india-rubber 

 tube, the one to the moveable glass reservoir G and the other to 

 GT, Fig. 9. These reservoirs are supported in sliding frames by 

 means of friction springs, and may be raised and lowered at 

 pleasure. The upper extremities of the voltameter tubes are cut 

 smooth and left open, but weighted levers, L and L', are provided, 

 with india-rubber pads, which usually press down upon the open 

 ends, closing them, but admitting of their being raised, with a 

 view of allowing the interior of the tubes to be in open communi- 

 cation with the atmosphere. Having filled the adjustable reservoirs 

 with dilute sulphuric acid, on opening the ends of the voltameter 

 tubes, the liquid in each tube will rise to a level with that of its 

 respective reservoir, and the latter is moved to its highest position 

 before allowing the ends of the tubes to be closed by the weighted 

 and padded levers. 



The ends of the platinum wire forming the electrodes may be 

 platinized with advantage, in order to increase the active surface 

 for the generation of the gases. 



PYROMETER AND VOLTAMETER CONNECTED. Figure 10 repre- 

 sents the connections of the voltameter with the pyrometer, and also 

 shows the necessity for the third leading-wire referred to at p. 165 

 in the Second Part of this Paper. One electrode of each voltameter 

 is connected with a common binding screw, which latter may be 

 united, at will, to either pole of the battery, whilst the remaining 

 two electrodes are, at the same moment, connected with the other 

 pole of the same battery ; the one through the constant resistance 



