The following observations on the electrical storage of energy are gathered from Prof. Oliver Lodge's Cantor lecture on "Secondary Batteries and the Electrical Storage of Energy."

Methods of storing energy are very numerous, and may be divided into 2 classes, mechanical and chemical. Under the first class come the raising of weights, as by the pumping of water into a reservoir; and that is a very efficient method of storing energy for future use, as a large proportion of the energy can be made available. Another mechanical method is by the coiling of springs, or producing strain in an elastic body by twisting - a form of storage familiarly exemplified in winding up a watch. A third form of mechanical storage is by charging a Leyden jar. The old idea of the action of a Leyden jar was that it was a chemical action; but that is not the case. What is being done in charging a jar is producing a strain on the molecules of the glass vessel. If the strain be carried too far, the glass will burst under the internal tension, producing a disruptive discharge, and overcoming the molecular resistance. There are also various chemical methods of storing energy. One class is represented by gunpowder, dynamite, and other explosives, though it must be confessed that this use of the term chemical storage is somewhat questionable in the case of gunpowder, for in it are combined substances not in themselves explosive, which in combination yield great energy when a light is applied to them.

The smelting of metals is a better example of chemical storage. By the melting of zinc is produced a material which can be employed for evolving energy in a common battery. Then we have the energy of sunlight stored on a gigantic scale by radiation upon vegetable substances, and this stored solar energy can be reproduced by the burning of coal ei her directly or as gas. This form of energy is utilized as a mechanical power in the well - known gas - engine, in which a mixture of coal gas and common air produces a mild form of explosion which furnishes an available energy. The amount of energy obtained for a given quantity of gas by this electro - chemical decomposition is not very great; other gases are known which have far more explosive power in small quantities than a mixture of coal gas and air. Thus a mixture of oxygen and hydrogen, if brought together, will decompose and explode with far greater violence than coal - gas and air, and thus can be made to store and reproduce a larger amount of energy.

Many persons are directing attention to the second example of decomposition of gases as a possibly practicable method of storing and reproducing energy, and Prof. Lodge is not quite sure but that it will be rendered useful in the future: whether it ever will be found an economical manner of utilizing energy remains to be seen; most probably not. Even in a test - tube the gas is too much confined to evolve much energy. These forms of storage by mechanical and chemical means may be distinguished as proper or improper, and in more exact language, as homo - tropic and heterotropic.

Passing next to consider the direct storage and reproduction of electric energy, Dr. Lodge points out that this can only be effected by the use of 2 conducting surfaces or plates, one of which must be different either in material or in action from the other. One of these plates must be attackable by electric energy, and the other not so attackable. Any difference in the attack - ability of the 2 plates will cause them to act as a battery - and the action may be produced either by using plates of different metals, or by acting in a different manner on 2 plates of the same material. In a platinum cell, one plate is oxygenized and the other hydrogen - ized. The hydrogenized plate is the one attacked, and the oxygenized plate is not attacked; the current passes through the cell from one plate to the other till the gas stored on the plates, i.e. the energy, is consumed, an event which happens very soon. A secondary battery or store of electric energy consists of a reservoir connected by wires on the one side with a plate of source, and on the other with a plate of use, and some switch arrangement is needed to connect or disconnect it as required with the plates of source and use.

A simple example of a secondary battery consists of a store or glass cell containing dilute sulphuric acid, in which 2 plates of lead are partially immersed, and these are connected by wires at will by a switch, either with a bell or a galvanometer, or with both. After the plates have been immersed a few seconds, it is found that they are saturated with electricity, as shown by the escape of gas in bubbles, and any attempt to store energy by continuing the current beyond that point is sheer waste. By switching it on to a bell and galvanometer, it can be seen how transient the energy in such a cell is, as it only rings the bell or deflects the needle for a few seconds, and the current is then exhausted, the cell needing a few minutes' rest to recover its energy. This is because the 2 plates cannot retain the gas; what is wanted is a plate to hold the oxygen, and become oxidized. This is the principle of Grove's gas battery. In Hitter's secondary piles, oxidized metals are employed; but it is necessary that the oxide of the metal used for plates shall not be soluble. Lead fulfils this condition, not being soluble, and is therefore a good material for plates, but it absorbs more than it ought.

Silver and manganese, like lead, have a peroxide; but lead is better suited for use as plates than other metals, because its peroxide is less soluble in dilute sulphuric acid, and hence one can store in leaden plates the largest quantity of electric energy. In defining the positive and negative poles, Dr. Lodge calls that the positive plate which is oxidized, and from which the current enters the store; and that the negative which is hydrogenized, and by which the current leaves. In other words, the oxide plate is plus or positive, and the hydrogenized plate minus or negative. Whether tested by duration of ringing or deflection of needle, the negative plate is the first to fail. This was the condition of the Ritter secondary pile before the days of Plante. The improvement introduced by the latter worker consisted in providing a reducible plate. The lead in itself is irreducible. Plante achieved his object by the simple process of reversing the action in the cell, converting the positive into the negative plate, and vice versa, so that the hydrogenized plate became oxidized.