July 13, 1882] 



NATURE 



251 



tion was a little slower than that of the movement, so that 

 each exposure should show a phase one step in advance 

 of the previous ones ; or, again, the rapidity of the periods 

 or that of the motion maybe such as to make it necessary 

 to expose only at each second, third, or longer periodic 

 interval. This would be effected by the use of two discs 

 rotating at different velocities. Suppose, for example, 

 one to revolve three times while the other revolved twice, 

 then the two slits would be in accord in front of the eye- 

 hole only once in three revolutions. 



In order to present the images formed by two succes- 

 sive glimpses as simultaneous pictures seen side by side 

 in the field of view, I took a prismatic eyeglass of the sort 

 sold by spectacle-makers to correct want of parallelism in 

 the optical axes. I cut it in two pieces, and placed these 

 in opposite ways in front of two horizonal slits, lying one 

 above the other in a shutter that fell vertically between 

 slides. When the first slit came in front of the eye, the 

 image it transmitted was deflected four degrees to the 

 left ; and when the second slit followed it, its image was 

 deflected four degrees to the right, and two apparently 

 simultaneous pictures were produced. Also, by crossing 

 the prisms I found it would be easy to construct an ap- 

 paratus with four successive slits shewing four images ; 

 1, up to the left, 2, up to the right ; 3, down to the left, 

 and 4, down to the right. I doubt, however, whether 

 this would be often found a useful development of the 

 instrument, owing to the difficulty of watching more than 

 a small area with attention. 



I noticed an important optical effect, namely that the 

 image first seen was always considerably fainter than the 

 others, showing that its brightness had faded in the brief 

 interval that elapsed before comparison began. It would 

 appear that the law of the rate of fading could be investi- 

 gated by this apparatus. I have not now the opportunity 

 of dcing so myself, but if I had, I should mount two 

 prisms below radial slits in a disc that was revolving 

 steadily at a known velocity, and I should watch a cir- 

 cular wafer through them. The width of the slits would 

 be adjustable, and so would the angular distance of the 

 prisms, and I should measure under various circumstances 

 the width of the second slit that was necessary to tone 

 down its image to an equal brightness with that seen 

 through the first. Or the investigation might be made 

 without prisms, by using two wafers and watching them 

 with the same eye through slits at different radial dist- 

 ances, separated by various angular intervals, the adjust- 

 ments being such that only the outside wafer should be 

 seen through the outer slit, and the inside wafer through 

 the inner one. Francis Galton 



THE CHEMISTRY OF THE PL ANTE AND 



F All RE CELL 



Part III.— The Discharge of the Cell 



r THE two plates of a Plants or Faure battery consist 



•*■ essentially of lead peroxide as the negative element, 



and metallic lead in a spongy condition as the positive. 



These are brought into communication with one another 



through the lead plates which support them, together 



with the connecting wire. 



The lead peroxide reacts both with the lead plate that 

 supports it, and with the lead on the opposite plate. At 

 first sight, it might be expected that the reaction between 

 it and the supporting plate would be the greater, as the 

 space between them is so small, and the resistance of the 

 intervening liquid in consequence almost inappreciable. 

 The action is, indeed, probably greater at the first moment, 

 but, as explained in our first paper, sulphate of lead is 

 immediately produced, and that which lies at or near 

 the points of junction, forms no doubt a serious obstacle 

 to further local action, and admits of the lead on the 

 opposite plate coming more fully into play. 



If we consider a priori what is likely to be the reaction 



between lead peroxide and lead, with water as the con- 

 necting fluid, we should expect : — 



Pb0 2 I H 2 I H 2 I Pb = PbO | H.O | PbH,0 2 . 



On experiment this is found to be actually the case, 

 yellow oxide appearing on the negative plate, and white 

 hydrate on the positive. 



If, however, the reaction takes place in presence of 

 dilute sulphuric acid, the result will inevitably be sulphate 

 on both sides, for even if oxide be first formed, it will be 

 attacked by that acid. Of course this production of lead 

 sulphate on each side might be expected gradually to 

 produce a perfect electrical equilibrium. This, in fact, 

 does take place under certain circumstances, but not under 

 others. The reaction on the negative plate is always 

 of this character, as far as our analyses have shown. 

 We have invaraibly found the deposit to consist of sul- 

 phate of lead mixed with unaltered peroxide. If, how- 

 ever, the cell be allowed to discharge itself rapidly, the 

 lead on the positive plate is converted, not only into the 

 sulphate, but, very partially, into lead peroxide. This is 

 sometimes evident to the eye from the puce colour of the 

 superficial layer, and we found also that this was con- 

 firmed by several chemical tests. 



It is difficult to conceive how the reduction of the 

 peroxide of lead on the one plate to oxide or sulphate, 

 should be attended by a direct oxidation of lead on the 

 other plate up to peroxide itself, as that would involve a 

 reversal of the electromotive force. It is more easy to 

 imagine that the peroxide results from the oxidation of 

 sulphate of lead already formed, through the agency of 

 electrolytic oxygen. 



When this peroxide is formed on the positive plate, it 

 is not difficult to foresee what must happen. A state of 

 electrical equilibrium will be approached before the perox- 

 ide of lead on the negative plate is exhausted. But the 

 two sides are in very different positions with regard to 

 local action. ' On the negative plate, the peroxide being 

 mixed with a great deal of lead sulphate, it will suffer 

 decomposition only very slowly through the agency of 

 the supporting plate, but the lead peroxide on the positive 

 plate, being mixed not only with lead sulphate, but with 

 spongy metallic lead, will be itself speedily reduced to 

 sulphate. Hence, on breaking the circuit, when local 

 action alone can take place, the peroxide formed on the 

 positive plate during the discharges will be destroyed 

 much more easily than the original peroxide on the other 

 plate. The difference of potential between the plates 

 will be restored, and on connection the cell will be again 

 found in an active condition. 



Now it has been frequently observed that partially dis- 

 charged accumulators do give an increased current after 

 repose, that is, after the circuit has been broken and 

 re-established. It remained for us to ascertain whether 

 the chemical change above described coincided in any 

 way with the physical phenomena. For this purpose we 

 prepared plates according to the method of Faure, and 

 examined carefully the changes of electromotive force 

 and strength of current, which took place during their 

 discharge under known resistances, and the chemical 

 changes that took place under the same circumstances. 



We found that the initial electromotive force of freshly 

 prepared cells was 225, 225, 2'2i, and 2-31 volts, aver- 

 aging 2-25, but that after standing for thirty minutes or 

 so, or when allowed to discharge for a few minutes, it 

 was reduced to about 20 volts. We take this to repre- 

 sent the normal electromotive force of the arrangement 

 of lead, lead peroxide, and dilute sulphuric acid, and 

 believe that the higher figure obtained at the first moment 

 is due to the hydrogen and oxygen occluded on the 

 respective plates, and which either diffuse out or are 

 speedily destroyed. 



We found, however, that in the discharge the electromo- 

 tive force diminished under certain conditions. Thus, in 



