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Rh that both strong and weak acid have much higher resistances than discharge was due to electrolytic hydrogen and oxygen simply. the liquid usually employed in accumulators, and it is therefore He ignored the chemical action of the acid. In 1882 Gladstone reasonable to suppose that local variations in strength of acid and Tribe published a series of papers, showing, by analyses, that cause the changes in resistance. That these are not due to the sulphate ol lead was formed on both plates, the action being, before discharge,

+ plate. Liquid. - plate. y Pb02 + + 2. Pb _ + n. H2Oj and after discharge -f plate. Liquid. - plate. r(a;-p)Pb02-| r(y-2p)H2S04-| r(z-p)Pb 1 L+i?. PbSOj L (» + 2p)H20 J + L+p. PbS04J

These results are in harmony with Gladstone and Tribe’s analyses,which showed that in every case some of the active material remained unchanged. During charge, the substances are restored to their original condition, and the equation must be reversed.

The theory received abundant confirmation—from Frankland in 1883, Reynier (1884), Crova and Garbe (1885), Tchelltzow (1886), Heim and Kohlrausch (1889), Ayrton, &c., with Robertson (1890>, Dolezalek (1897), and Mugdan (1899). Notwithstanding this body of evidence it has been objected to, chiefly on the ground that sulphate of lead is a white substance and is difficult to reduce to lead, whereas the substance formed in a cell is not white and is easy to reduce. But Gladstone and Tribe (and afterwards Swinburne) showed in 1883 that sulphate was easy enough to reduce when mixed with other substances like peroxide, or lead itself, which is just the condition in which it occurs in all normal working of the cells. Darricus suggests that sulphate is formed F/g 29. only on the negative plate, oxide of lead on the positive, and that the spongy lead is in an allotropic condition, which helps to give A constitution of the plugs is shown by the fact that, while the plugs higher This theory is contrary to the results obtained are almost identical at end of discharge and beginning of charge, by the e.m.f. numerous experimenters already mentioned. Mugdan has the resistance falls from 0'0055 to 0'0033 ohm.

While a current2 Hows through a cell, heat is produced at the rate especially shown that oxide cannot be detected on the positive _ of C R x ‘24 calories (water-gram-degree) per second. a tureofcell. consequence the temperature tends to rise. But the change of temperature actually observed is much greater during charge, and much less during discharge, than the foregoing expression would suggest ; and it is evident that, besides the heat produced according to Joule’s law, there are other actions which warm the cell during charge and cool it during discharge. Messrs. Duncan and Wiegand {loc. cit.), who first observed the thermal changes, ascribe the plate. It is doubtful if the experiment in favour of an allotropiechief influence to the condition of the lead can be confirmed. The body of evidence in electro - chemical addi- favour of the formation of sulphate on both plates is exceedingly tion of H2S04 to the strong—in fact, decisive. Figs. 31 and 32 embody a complete Serb # liquid during charge of chemical and physical observations ; many of the precedii g" 77/77© in Hoursorfrom beginning of Charge and its removal during statements are illustrated by these curves and can be tested by Discharge discharge.

Fig. 30 their relationships. The current was stopped at the points marked gives some results ob- A, B, C, D, to take active material for analysis ; and the rise of Fig.30.

tained by Ayrton, e.m.f. in discharge, and fall in charge, during this short interval is Lamb, &c. This elevation of temperature (due to electrolytic noteworthy. strengthening of acid and local action) is a measure of the energy (2) Plante attributed the high charging e.m.f. to hydrogen lost in a cycle, and ought to be minimized as much as possible. dioxide, and similar oxygenized bodies. Gladstone and Tribe thought that occluded hydrogen and oxygen gases, though very The chemical theory which has been adopted in the small in quantity, might account for it. Robertson thought that foregoing pages is very simple. It declares that sulphate the oxygenized bodies arising from persulphuric acid might raise Chemistry ^ea(^ formed on both plates in discharge, the chemical action being reversed in charge. Most of the other actions — associated with variations in the e.m.f. of the cell —arise from local variations in strength of acid brought about by chemical action and the electrolysis of the acid. It will contribute to a clear understanding if the chemical actions be considered under various heads :—(1) Those occurring during ordinary charge and discharge ; (2) the cause of high e.m.f. at end of charge; (3) the rapid fall of e.m.f. and p.d. at end of discharge; (4) the rapid recuperation of a discharged cell when allowed to repose; (5) the effect of repose on a charged cell; and (6) the agreement of the experimental e.m.f. and that calculated by the Kelvinor lower the e.m.f. according to their exact position in a cell. If) Helmholtz thermodynamic equation. however, these substances are added to a cell (without changing: (1) Plante thought that the chemical action during charge and the acid strength) they do not produce a change at all comparable