If we have not been able, however, to form a Vol taic apparatus absolutely dry, and possessing a strong power of decomposition, we may, at lead, obtain one whose action, though in truth very weak, is of very long duration; such is the pile which Mr Hachette has constructed with pairs of metallic plates, separated by a simple layer of farinaceous paste, mixed with marine salt. When this layer is dried, the moisture which it attracts from the atmosphere renders it sufficiently conducting, to admit the re-establishment of the elec trical equilibrium among the metallic elements, in a period of time quite imperceptible. It then charges the condenser by a simple contact, to our senses in stantaneous; and it preserves this property for whole months and years, which renders it a real electro phorus ; but it excites neither shock nor taste, nor chemical action. Mr Zamboni has also constructed a pile, the electrical effect of which appears to be very durable. He composes it with discs of paper, gilt or silvered on one of their sides, and covered on the other with a layer of pulverized oxide of manganese. In the superposition of these discs, then, the pairs of metal plates are formed of silver or gold in contact with oxide of manganese, and the interposed paper serves as the conductor. Hence arises a very weak trans mission of electricity. With this system we obtain signs of the electrical influence in the same manner as with the pile of paste; but neither chemical action nor shock, nor even taste. This last class of phenome na, .then, requires a more rapid re-establishment of the electrical equilibrium ; and to demonstrate the extreme effects of its retardation, Mr Riot construct ed piles in which discs of nitrate of potash, melt ed by heat, were substituted for the moistened body ; then the conductibility was so weak, that the con denser took a sensible time to charge itself, and continued charging more and more until a certain limit, which was the same as with the most power ful piles having the same number of pairs of plates. From the observed law of these charges, we may conclude, that the initial quantity of electricity com municated by such a pile to the condenser, in an infinitely small portion of time, is incomparably less than with the ordinary piles ; and, as it is these initial charges which produce chemical de compositions when the communication is formed between the two poles, we may understand why these piles, where the conductibility is very weak, do not produce these phenomena, and excite neither chemical action, nor taste, nor shocks. This con sideration of the initial velocities affords a very sim ple explanation of a great number of phenomena, apparently very puzzling. They show, for example, why the apparatus with cups, filled with a weak acid, exerts, at the moment it begins its action, a very intense power of decomposition, which is quickly weakened ; and, after an inconsiderable interval of time, seems almost extinct, though the condenser ap plied to its columns is always charged by a single contact, with the same quantity of electricity. This is owing to the contact, although very short, not being altogether instantaneous. It may seem so to us, although the rapidity of the charge, during the instant it lasts, may have suffered enormous varia tions. The final equality of this charge, then, af fords no information as to the progressive law by which it has been formed ; and does not prove that this law is similar or different. But the more or less intense power of decomposition produced by the electrical current is a much clearer proof of its rapidity ; because these decompositions depend at once on the absolute quantities of electricity trans mitted, and of the rapidity with which it is succes sively furnished by the apparatus, in proportion as the circle of conductors through which it passes dis charges it continually.
This unequal velocity of the electric current, in different Voltaic piles, or in the same apparatus at different periods, may be rendered in a manner pal pable by the following experiment : Having form ed a pile where the conductors are layers of farina ceous paste, insulate it on a cake of resin, and make its two poles communicate by means of a prism of al kaline soap, in the middle of which the two conduct ing wires attached to these poles are sunk, in such a manner that their points of insertion may be always asunder. The soap will now be seen to conduct the
electricity sufficiently well to discharge completely the poles of the pile, in proportion as they are re charged by the decomposition of the natural electri cities of the discs ; for all electrical tension will com pletely disappear from these poles ; and, if a con .
denser be applied to them, it will not charge itself in any degree, whether the pile be insulated, or the communication be even formed, through the medium of the most perfect conductors, between the soap or the discs and the ground. But, if the same piece of soap be interposed between the two poles of a pile of equal tension, constructed with a good liquid con ductor, such, for example, as a solution of muriate of soda, it will not be capable of completely dis charging it as fast as it is recharged. There will remain always a degree of electrical tension at each of its poles, which may be capable of charg ing the condenser. Although these two piles, therefore, may both attain the same degree of final charge, and the same degree of repulsive force at their poles, the total quantity of electricity which they put into circulation in a given time may yet be different, may even be incomparably greater in the one than in the other, and may thus render the one capable of producing chemical decompositions, which it is absolutely beyond the power of the other to ef fect.
Confined by the object of this article to what con cerns the electromotive apparatus itself, it does not belong to us to explain, in detail, the brilliant dis coveries to which it has given rise, when employed as the agent of chemical decomposition by Messrs Hisinger and Berzelitul, and, above all, by Sir Hum phrey Davy. We cannot, however, resist giving here, at least, an idea of these important results.
We have already seen, in the article the s'Aigular power which the Voltaic apparatus pos sesses of separating the constituent principles of wa ter. This experiment, a thousand times repeated, has been elaborately studied in its details, and has led to conclusions very useful in respect to other chemical decompositions. We shall, for this reason, therefore, first of all describe this process. The most conve nient apparatus for doing it well, seems to be that which has been contrived by Messrs Gay-Lussac and Thenard. It is represented at fig. 16, Plate LXXX.* E E is a glass funnel, the mouth of which B is closed by a stopper coated with sealing-wax, across which two wires of platina are made to pass paral lel, and distant from each other nearly half an inch ; these wires rise within the funnel an inch and a half, or two inches, above the bottom of it. Water is •then poured into the funnel, and each wire is covered by a small glass tube sealed in the top, and also filled with water. The external extremities of the wires are then made to communicate each of them with a pole of the pile, and the apparatus is arranged. Af ter it has acted for some time, the communication between the two poles is interrupted, and mea suring the volume of the gas disengaged under each covered glass, we there find twice as great a volume of hydrogen as of oxygen. These are, in fact, the proportions which constitute water ; for, on re-establishing the combination, there remains no gaseous residuum ; at least, when the water expos ed to the electrical current has been previously deprived of its air, and is preserved from the con tact of this fluid during the operation, which may be done, either by covering the funnel with a cover properly luted, or in placing it in a va cuum. Without this precaution the gases disen gaged by the pile would mix with portions of at mospheric air, either previously contained in the water, or absorbed by it during the operation • so that the nature and the proportion of the product would be altered by these circumstances. But, be sides this, in order to lose nothing of the action of the pile, the communication of the decomposing wires with the extreme elements must be perfectly esta blished ; and nothing is more convenient for this pur than plunging them into a little cup of glass pose, with mercury ; in which are plunged two thick wires of iron, cemented to the extreme plates of the electromotive apparatus.