June io, 1892.] 



SCIENCE. 



325 



afterwards rapidly to 2.4 volts, or even higher. Upon dis- 

 connection of the charging current the potential difference 

 drops suddenly to about 2 1 volts, and then on discharge 

 falls rapidly to 1.95 volts. The main part of the discharge 

 takes place between 1.95 and 1.9 volts, and if it be continued 

 beyond the latter point the potential difference rapidly falls 

 to 1.6 volts, the gradient below 1.8 volts being very steep. 

 Last week, and again yesterday. Dr. Gladstone showed the 

 Institution of Electrical Engineers that the variations in the 

 strength of the sulphuric acid are the main causes of the varia- 

 tions in the electro motive force. Starting with a properly 

 formed cell which has been discharged, there are two leaden 

 supports; on one of these is a mixture of lead sulphate 

 (PbS04) with more or less lead peroxide (PbOg); on the 

 other is also a mixture of lead sulphate with more or less of 

 spongy metallic lead. Each of these mixtures is a porous 

 layer. The act of charging converts the lead sulphate on 

 one plate into PbOg, and on the other into spongy lead. In 

 the operation there is an abundant formation of sulphuric 

 acid in the pores of each plate, while an equivalent amount 

 of water disappears. In addition to this chemical effect 

 sulphuric acid is, by electrolytic action, heaped up against 

 the positive (peroxide) plate, and withdrawn from the neigh- 

 borhood of the negative (spongy lead) plate. The increase 

 of acid strength around the positive plate was proved experi- 

 mentally by the author, while it is matter of common knowl- 

 edge that the density of all the liquid in a cell rises during 

 charge. 



When a cell is fully charged and left to stand, the strength 

 of the acid commences to equalize itself through the liquid. 

 This is brought about by three causes — diffusion, local 

 action, and reduction by HgOj. These actions occur at the 

 positive plate, where the acid in the pores works out, at first 

 rapidly and then more slowly. At the same time energetic 

 local action is set up between the PbOg and its supporting lead 

 frame, with the formation of sulphate of lead, and the con- 

 sequent absorption of sulphuric acid from the liquid. The 

 temporary evolution of oxygen gas from a well-charged plate 

 has been attributed to the reaction of hydrogen dioxide on 

 peroxide of lead. At the negative plate equalization of acid 

 strength takes place by diffusion, and also by a direct, slow, 

 chemical action of the sulphuric acid on the lead, producing 

 lead sulphate and hydrogen gas. This latter gas, being formed 

 in the pores of the spongy lead, chokes them and hinders the 

 diffusion of the acid, rendering it very slow. 



During the discharge of a cell all the causes just enumer- 

 ated as tending to produce equalization of acid strength, 

 continue in operation, and to them is superadded the ordinary 

 discharge reaction of the cell. At the positive plate the lead 

 peroxide, with sulphuric acid existing in its pores (PbOg -\- 

 H3SO4), becomes sulphate of lead and water (PbS04-|-H30). 

 At the negative plate spongy lead with sulphuric acid in its 

 pores (Pb -|- H3SO4) also becomes sulphate of lead and 

 water (Pb SO4 -j-HgO) Further, by electrolytic action 

 sulphuric acid is transferred from the PbOg to the Pb plate. 

 The excess of acid originally about the PbOg plate rapidly 

 disappears by these various agencies, and the acid on both 

 plates is reduced pretty nearly to the same strength as that 

 of the intermediate liquid. After this there is a gradual 

 withdrawal of acid from the liquid in the pores, more or less 

 compensated by diffusion inwards from the intermediate 

 liquid. This brings about the reduction in the strength of 

 the whole acid, which is well known to take place during 

 discharge. The strength of the acid in the pores will be de- 

 termined by the relative values of the rate of withdrawal 



and the rate of diffusion. But while the rate of withdrawal 

 continues constant for a given current discharge, the rate of 

 diffusion rapidly diminishes. The rate of weakening of the 

 acid is, therefore, a constantly increasing one, and may 

 finally become so rapid that the acid strength of the liquid 

 against the working surfaces of the plates is very low, or 

 almost nil. 



It being shown that the strength of the acid against the 

 plates of a secondary battery is constantly varying during 

 charge, repose, and discharge, the authors of the paper, from 

 which we have quoted, set themselves to prove experimen- 

 tally that a change of electro-motive force is produced by a 

 change in the strength of the acid. Taking a pair of fully 

 formed and carefully washed plates they were placed in a 

 series of solutions of gradually increasing strength of acid, 

 and left in each for fifteen minutes. The acid strengths and 

 electro-motive force are given in the following table: — 



In a second set of experiments the Pb plate was kept in 

 acid of 14.0 per cent strength, while the acid around the 

 positive plate was varied from 6.5 to 81 per cent. The result 

 confirmed those of the first set of experiments, but it was 

 shown that the electro-motive force depends on the strength 

 of the acid at both electrodes. Several other series of ex- 

 periments were made in different ways, but all confirming 

 the opinion that change in acid density was accompanied by 

 a change of electro-motive force. 



We have not space to follow Messrs. Gladstone and Hibbert 

 through the vast amount of confirmatory evidence they ad- 

 duced from their own experiments, and fi'otu the records of 

 the researches of others, in support of their hypothesis. We 

 may, however, notice one point. Applying Lord Kelvin's 

 law as to the relation between the electro-motive force of a 

 cell, and the thermal value of the chemical actions con- 

 tributing to it, they find that the voltage of a PbOo — Pb 

 cell, in which there was nothing but pure H3S04, would be 

 2.627; by experiment they made it 2.607 volts. With pure 

 water in the cell the result is, by calculation, 1.35 volts; by 

 experiment, 1.36 volts. In charging an accumulator the 

 current has, as already shown, to do extra work in concen- 

 trating H3SO4 at the PbOg plate, and the energy equivalent 

 to that work must be obtained from an increased potential 

 difference. This explains how it is that potential difference 

 is so much greater during charge than during discharge. 

 For a dyad gramme equivalent of H3SO4, concentrated from 

 a 10 per cent solution to 100 per cent, about 17,000 calories 

 will be needed, equal to .37 of a volt. The calculated 

 charging electro-motive force must, therefore, be at least 

 2.3 volts. 



The lesson to be learned from the paper is the desirability 

 of promoting diffusion in the liquid of the cell, so as to keep 

 the whole of the same density. At present the heavy acid 

 slides down the PbOo plate and accumulates at the bottom. 

 This leads to differences of current density in different parts 

 of the plate, and will also give rise to potential differences 



