evident that in this pile the amalgamated zinc is outside the carbon: it is a hollow cylinder plunged into the glass vessel that is filled with diluted sulphuric acid: the porous tube is placed in the interior of the zinc cylinder, and itself receives the carbon and the nitric acid into which the latter must be plunged.

The preparation of the carbon is difficult when hollow cylinders, such as in Bunsen's pile, are in question. For this purpose, it is necessary to have iron moulds, and then coke in fine powder, which is brought by one or two operations to a high temperature after having been mixed with sugar or molasses to cause a cohesion, that gives consistence to the whole. In Bonijol's pile the cylinders may be prepared of carbon in the same manner, which is the easier, as they are solid. But it is a much more simple plan to procure pieces of well-baked coke of good quality and of sufficient dimensions. They are cut, as well as may be, into the form of cylinders, without, in truth, its being possible to give them exactly this form, which, however, is of little importance.

M. Deloeil is at the present moment constructing in Paris piles of this kind, in which the carbon is perfectly cylindrical, and has been prepared according to the process mentioned. A very simple arrangement enables us beforehand to establish a contact between the carbon or zinc of each pair; and to attach the pairs themselves upon fixed frames in such a manner that, in order to put the pile in action, it is only necessary to raise a wooden table that sustains the vessels filled with their liquids, and into which the carbon and the zinc are to be plunged, each in that which appertains to it.

Such are the different kinds of piles that are generally used, and all of which are more or less to be found in physical cabinets and in chemical laboratories. There are, however, some others that are used only in certain practical applications, and of which we shall speak again, when we are engaged in these applications.

We will not occupy ourselves farther at present with certain voltaic combinations, which have a purely theoretical interest, and the description of which will find its place when we are

treating upon the theory of the pile. However, we will mention here, as it is very generally used, Smee's pile, the pairs of which consist of amalgamated zinc and plates of platinised silver, namely, silver covered with a coating of the black powder of platinum, plunged into one and the same liquid, diluted sulphuric acid, and which, notwithstanding, preserve a remarkable constancy.

Enumeration of the different Voltaic Piles, and the general Consideration that is common to them all.

To sum up, the voltaic piles that we have described

are:

1. Volta's column pile, celebrated because it was the first, and because it represents the form under which its illustrious inventor realised his idea;

2. The wooden trough pile with fixed metal pairs,—the form under which was constructed the great pile given in 1806 to the Polytechnic School by the Emperor Napoleon;

3. The pile, with independent cells of glass or porcelain and movable metal pairs,— the form under which the pile of 2000 pairs belonging to the Royal Institution of London was constructed, and which was used by Davy in making the great discoveries that have immortalised his name;

4. Berzelius's pile, with copper cells, which was of essential service in the first experiments upon electro-magnetism;

5. Daniell's constant pile, with porous organic diaphragms, and two liquids, sulphate of copper for the copper, and acidulated or salt water for the zinc, a form eminently useful in experiments of long duration, and in particular those in which the study of the chemical effects of the current is concerned.

6. Grove's constant pile, in which, as in that of Daniell, two liquids and a diaphragm are employed, with this difference, that platinum supplies the place of the copper, and nitric acid the place of sulphate of copper; and the porous diaphragm is of unglazed not glazed earthenware ; a pile, the

best suited of all for the production of the effects of the electric current, on account of its enormous powers, united with a constancy, which, although less than in Daniell's pile, is sufficient in the majority of cases, and especially in public lecturing.

7. Bunsen's constant pile, which is only a modification of Grove's by the substitution of carbon for platinum; it is constant for a longer time, but is less energetic in its effects than Grove's. It is also very much used, especially in Germany, and it is found, as well as Daniell's, both in the workshop of the artizan and in the cabinet of the savant and the professor. Bonijol's pile and Deloeil's differ from Bunsen's only in the form and place given to the carbon.

This is the place to offer a general remark, which is common to all piles; and the importance of which will not escape those who employ these apparatus. It is to know accurately what determines in each of them the place of the positive pole and that of the negative. The employment of certain faulty denominations, joined to prejudices arising from certain theories, have cast upon this point, so simple in itself and so essential, an obscurity which ought not to exist, and which it is indispensable that we should dissipate.

There enter into the construction of all the piles of which we have spoken (which contain one or two liquid conductors), two solids of a different nature, the one more oxidisable than the other, almost always the zinc; the other less oxidisable, the copper, the platinum, the carbon. These solid bodies, which, for greater facility in explanation, we will suppose to be zinc and copper, are connected together two and two, so as to form pairs, which follow each other, and are separated from each other by the one or by the two liquids. Care is always taken to place the two solid bodies of the pair in the same order, so that, for example, in each pair every zinc is above the copper, if the pile is a column, or on the right of every copper if the pile is horizontal. The inverse order may equally occur; but it is necessary that the rank, whatever it may be, occupied by the two metals in respect of each other remain the same in all the pairs. It follows from this, that

in horizontal piles, which are the only kind now used, each cell contains, besides the liquid, the two solid bodies of a different nature, and never those of the same kind. With regard to the two extreme cells, one of them contains, besides the copper of the pair whose zinc is plunged in the preceding cell, a zinc to which a conductor is soldered; the other contains, besides the zinc of the pair whose copper is plunged in the cell that follows it, a copper, to which a conductor is also soldered. It is these two conductors that carry away the two electricities produced by the apparatus and accumulated at the two extremities or poles, and which are called the electrodes of the pile; one the positive, the other the negative electrode.

In the column pile the extreme zinc is also in contact with a copper, which is itself not in communication with any conducting liquid, and the extreme copper with a zinc that is in the same condition; but in the horizontal pile this copper and zinc are suppressed, as experience has shown them to be of no utility. This suppression does not, in the least, change the nature of the poles; so that whilst, in the column pile, a plate of copper is the negative pole and a plate of zinc the positive, in the horizontal piles the negative pole is found at the last zinc, and the positive at the last copper. From this circumstance some confusion has often arisen, especially when, instead of employing the words positive and negative, we have wished to designate the poles by the names of the metals, as the zinc pole and the copper pole. The nature of the electricity that is accumulated, must not be associated with the nature of the substance that terminates the pile at each extremity; for, as we have seen, this may lead to grave errors. It must be connected with the order according to which the solid substances are placed; and we must remember that the positive pole is always at that extremity of the pile towards which the zincs of each pair are turned; the negative pole at that toward which all the coppers are turned; and this, whatever be the manner in which the pile terminates, whether by a plate of zinc or one of copper. If the zincs of each pair are turned to the left of a person who is looking at

the pile, and the copper consequently to the right, the positive pole will be at that extremity of the apparatus which is to the left of the observer, and the negative at that which is at his right. If the pile, instead of being arranged in the same right line, returns upon itself, and is arranged in any direction. whatever, we have merely to see on which side the zincs are turned, and to remember that the positive pole is on that side; as also that the negative pole is on the side to which the coppers are turned. In this way we are sure never to

make an error.

Finally, it is well to know that, for dynamic effects, a pile composed of a single pair may be sometimes employed: thus, Mr. Hare's first helix pile (Fig. 30.) consisted of but a single pair with large surfaces, and, as this pair was eminently fitted for producing the effects of the incandescence of wires, the author called it a Deflagrator. In the case of a single pair there is but one cell, into which are plunged separately the two solid bodies which are not in immediate contact. From each of them proceeds a metallic conductor; and the conducting substance placed between these two conductors is traversed by an electric current. If this substance is fine

wire, it is heated, and it can be made red hot if the pair is sufficiently powerful; if it is acidulated water, it is decomposed; but this latter phenomenon cannot occur unless the single pair is one of Grove's or Bunsen's. In the case of the decomposition of water by a single pair, the hydrogen is seen to accumulate around the platinum wire coming from the zinc of the pair, and the oxygen around that proceeding from the platinum or the carbon. Thus the negative electricity comes from the zinc, and the positive from the platinum or the carbon. This equally takes place, as we shall presently see, in a pile of many pairs, if care be taken to suppress the copper in contact with the last zinc and the zinc in contact with the last copper, which Volta had thought necessary, and which occurred in his column pile, but which, as we have said, are perfectly useless, and never exist in the trough piles.

It is important to bear in mind, when occupied upon a pile