Connections must oe provided to the sheets in either form, and the best is - made by strips of lead attached carefully to the sheets by soldering, which should be well protected by good cement; copper wire should not be used, as it is sure to be acted upon, - and form salts, which will exert a very mischievous action.

Containing vessels may be of any suitable material, but glass has the great advantage of permitting the action to be watched; if leaden, or wood lined with cement, or other opaque vessels are used, they should be covered with sheet glass for this reason. They must not be entirely closed, because gases are generated, and must be allowed to escape; they should not be uncovered, in order to resist evaporation, and also for prevention of dirt, which would be likely to result in short - circuiting the plates - a thing very likely to occur, and obviously injurious to the working.

Space for acid must be allowed, sufficient to effect the action; therefore the distance of the plates must be so adjusted. Circulation cannot be depended on, but it is desirable to raise the bottom of the plates above that of the vessel, and to allow the liquid to rise above them, in order that the heat and escape of gas may tend to produce a current, and to draw the external acid between the plates; 1 lb. of lead requires 1/2 lb. of acid to convert it into sulphate, and as by th foregoing proportions there would be 1/2 lb. of lead to be converted on the 2 faces of lead opposing per sq. ft., this requires space for 1/4 lb. of acid, which, diluted as 1 to 10, would be contained in a space of 1/3 in. between the plates.

The strength of the acid varies during the action, becoming strongest when . the charge is complete, and one plate is converted into spongy lead and the other into peroxide; when discharge is completed, a great part of the acid is absorbed in the formation of sulphate of lead. Several consequences result: (1) the resistance of the battery is lowest just when its electromotive force is highest, and vice versa, which introduces a variation in the current generated at different periods of discharge; (2) when the material consists of a porous mass containing liquid confined among its interstices, the acid may be entirely removed at an early period of the discharge, and consequently much material remains unacted upon; also, the residuary liquid being highly resistant at the next act of charge, the current - cannot reach the material. As a consequence, a cell containing a large mass of active material may be able to do but little work. This defect is also an accumulative one : portions of the mass become practically non - conducting, and insulate other por tions to which the acid has access, because in very dilute acid, instead of the normal sulphate PbS04, there is a tendency to produce the basic sulphate PbS04 + PbO, which is not readily reduced by hydrogen.

To charge a secondary battery it is necessary to employ an E.M.F. greater than its own, and greater in proportion to the rate of charge desired. All such excess of E.M.F. is energy lost in overcoming resistance; therefore slow charge is most economical under this head, though other practical considerations have to be taken into account, i.e. against energy lost at the rate of the square of the current generated, must be reckoned time and interest on plant. But in addition to the loss of energy involved, a small charging current is desirable for 2 good reasons: (1) the product is in better condition, the particles being in closer contact and better electrical connection; (2) there is less loss by uncombined gases escaping.

Throughout the charging there is a constant escape of gases going on, chiefly oxygen; and the loss of either gas means total loss of the equivalent of electricity involved in the decomposition from which it arises. If 0 is wasted, H is lost too, or the total power of the cell is reduced by its incapacity to take up the 0. But the escape of H indicates either that the rate of charging current is too great, or that the limit of the economical charge is approached. The rate at which a unit area of surface can act properly, in the case of a secondary receiving charge, is a lowering capacity, because it is related to the diminishing quantity of lead sulphate remaining unconverted. The increase of free acid tends to increase the current, and the combination of these 2 causes results in a growing loss of gases as the charge proceeds. Obviously, therefore, it is bad economy to press the charge to the full capacity.

The electromotive force is about 2.25 volts immediately after charging, but falls spontaneously to 2 volts. This high initial force cannot be due to the free H and 0, because their force of combination is only 1.5, but it is easily accounted for by the presence of ozone, which is oxygen charged with a higher energy in order to force the third atom of 0 into the molecule. The normal E.M.F. of 2 volts is subject in working to a fall such as occurs in ordinary batteries to an extent increasing with the rate of current, and due probably to the change in the liquid particles adjoining the plates: therefore the E.M.F. rises again after a short rest has allowed fresh liquid surfaces to reach the plates by diffusion. The average rate of working E.M.F. is probably 1.9 to 2 volts.

Metallic solutions, while promising in appearance, do not answer in practice. The action, unless very slow, alters the layer of liquid in contact with the metal, which then refuses to act; in charging, after the first action, there is no metallic salt present to decompose, but only acid which gives off gas, and so the metallic deposit becomes noncoherent; in discharge, the metallic salt forms too rapidly to dissolve, and crystallizes on the plate.

Acid solutions other than sulphuric might be used, and no doubt will; but hydrochloric acid when electrolysed does not give up H and Cl simply - it is always accompanied with oxygen, and the result is the formation, not of chlorides, but of oxychlorides, which are exceedingly refractory in reduction; for this reason, the silver chloride battery fails in reversal, or it would constitute an admirable storage battery.