time, and has, therefore, to be frequently renewed; besides which, there is always a deposit of copper upon the paper, thus occasioning a loss.

Common coarse garden-pots answer excellently for porous vessels, closing the aperture at bottom by a cork.

The precautions that have been given (see Daniell's Battery, p. 39), as to the preserving of the porous cells when not in use, are applicable to the cells or partitions used in these processes, which, when not in use, should be kept in water, or should not be allowed to dry until they have been in water long enough to dissolve out the salts that were within the pores of the cell; otherwise the salts crystallize, and either crack the cell or cause it to scale off in small pieces. Porous cells, when not thoroughly washed and freed from salts, if laid aside for a few days, often thrown out an efflorescence, or crystalline growth, like mould, of a soft silky texture, and from one-half to one inch in length. An analysis of this efflorescent matter gave

COMPARATIVE VALUE OF EXCITING SOLUTIONS.-We have recommended the porous cell being filled by dilute sulphuric acid, which we consider best; but other saline solutions will serve the same purpose: solutions of common salt, sal ammoniac, sulphate of zinc, have been recommended, and each has been called best in its turn. The following results of experiments with these solutions in the porous cell will show their relative qualities, and enable the student to judge for himself. The size of the zinc plate in the cell used in these experiments measured 6 inches by 6 inches; the copper plates upon which the deposits were formed were the same size; the solution of copper was kept at the same strength; the time that each was in solution was 16 hours.

HOW OFTEN SOLUTIONS SHOULD BE CHANGED AND ZINC AMALGAMATED. Students have often put this question to us: How often should the solution in the cell be renewed, and the zinc plate be amalgamated? The following are the results of many trials made to ascertain the facts necessary to answer this inquiry. The zinc plates used were nearly one foot square, and the copper plate upon which the deposit was made was of the same size as the zinc plates. In the first series the zinc plates were not taken out either to brush or re-amalgamate, neither were the solutions renewed during the time specified.

From these results it is evident that the best and most economical manner of treating the solution and the zinc would be to renew the solution every 24 hours, as the second 24 hours do not give, without renewal, above half the deposit of the first 24 hours, while the waste of zinc is very little less than in the first.

The next series of experiments was with the same zinc and the same kind of solutions, but the zinc was taken out every 24 hours, and brushed, but not re-amalgamated, and put back again with new solution in the porous cell.

These results give the most ample reply to the question so often put, and will guide the manufacturer as well as the student in his operations, whether time or material be of the greatest consequence to him.

We may remark that the sulphate of zinc solution does not require renewal, but simply that we half empty the cell and refill it with water. The sulphate of zinc poured out being nearly saturated, may be crystallized, and will serve for other electro-metallurgical operations.

MAKING OF MOULDS.-The directions giving for obtaining a mould from a penny-piece, by deposition, are applicable to taking moulds from any metallic medal, engraving, or figure that is not undercut; and for depositing within the moulds so produced. On the first discovery of this art, the electrotypist was confined to metallic moulds, as the deposition would not take place except upon metallic surfaces; but the discovery that plumbago, or black lead polished, had a conducting power similar to that of metal, and that the deposit would take place upon its surface with nearly the same facility as upon metal, freed the art at once from many of its tranfmels, and enabled the operator to deposit upon any substance -wood, plaster of Paris, wax, etc.-by brushing over the surface with black lead. It obliged the electro-metallurgist, however, to render himself expert in the art of moulding, since no good electrotype can be obtained without a perfect mould. We shall, for this reason, endeavor now to give such instructions as will enable the student to make good moulds after a very short practice; but we need hardly add, that in this as well as in every operation, however plain may be the instructions and easy the manipulations, practice is necessary to ensure success; so that the student ought not to lose patience should his first attempt not succeed to his wishes. The substances used for taking moulds from objects to be copied by electrotype are beeswax, stearine, plaster of Paris, and fusible metal; recently, gutta percha has been very successfully used. The articles to be copied are generally composed either

of plaster of Paris or metal. Suppose, in the first place, the article to be copied is of metal, and a mould is to be taken from it in wax or stearine. The latter we have not found to answer well alone; when used it should be mixed with wax, about half-and-half.

PREPARATION OF WAX.-Whether the beeswax have stearine in it or not, it is best to prepare it in the following manner :-Put some common virgin wax into an earthenware pot or pipkin, and place it over a slow fire; and when it is all melted, stir into it a little white lead (flake white)-say about one ounce of white lead to the pound of wax; this mixture tends to prevent the mould from cracking in the cooling, and from floating in the solution: the mixture should be re-melted two or three times before using it for the first time.

TO TAKE MOULDS IN WAX.-The medal to be copied must be brushed over with a little sweet oil: a soft brush, called a painter's sash tool, suits this purpose well: care must be taken to brush the oil well into all parts of the medal, after which the superfluous oil must be wiped off with a piece of cotton or cotton wool. If the medal has a bright polished surface, very little oil is required, but if the surface be matted or dead, it requires more care with the oil. A slip of card-board or tin is now bound round the edge of the medal, the edge of which slip should rise about one-fourth of an inch higher than the highest part on the face of the medal: this done, hold the medal with its rim a little sloping, then pour the wax in the lowest portion, and gently bring it level, so that the melted wax may gradually flow over; this will prevent the formation of air bubbles. Care must be taken not to pour the wax on too hot, as that is one great cause of failure in getting good moulds; it should be poured on just as it is beginning to set in the dish. As soon as the composition poured on the medal is set (becomes solid), undo the rim, for if it was allowed to remain on till the wax became perfectly cool, the wax would adhere to it, and being thus prevented from shrinking, which it always does a little, would be liable to crack. Put the medal and wax in a cool place, and in about an hour the two will separate easily. When they adhere, the cause is either that too little oil has been used, or that the wax was poured on too hot.

ROSIN WITH WAX.-Rosin has been recommended as a mixture with wax; mixtures of which, in various proportions, we have used with success; but when often used, decomposition, or some change takes place, which makes the mixture granular and flexible, rendering it less useful for taking moulds. When rosin is used, the mixture, when first melted, should be boiled, or nearly so, and kept at that heat until effervescence ceases; it is then to be poured out upon a flat plate to cool, after which it may be used as de scribed.

MOULDS IN PLASTER.-If a plaster of Paris mould is to be taken from the metallic medal, the preparation of the medal is the same as described above; and when so prepared with the rim of card

board or tin, get a basin with as much water in it as will be suffi cient to make a proper sized mould (a very little experience will enable the operator to know this), then take the finest plaster of Paris and sprinkle it into the water, stirring it till the mixture becomes of the consistence of thick cream; then pour a small portion upon the face of the medal, and, with a brush similar to tha used for oiling it, gently brush the plaster into every part of the surface, which will prevent the formation of air-bubbles; then pour on the remainder of the plaster till it rises to the edge of the rim : if the plaster is good, it will be ready for taking off in an hour. The mould is then to be placed before a fire, or in an oven, until quite dry, after which it is to be placed, back downwards, in a shallow vessel containing melted wax, not of sufficient depth to flow over the face of the mould, allowing the whole to remain over a slow fire until the wax has penetrated the plaster, and appears upon the face. Having removed it to a cool place to harden, it will soon be ready for electrotyping. If the mould is large and the plaster thick, the wax may be put upon the surface, and only as much as will penetrate a small way into the plaster. In both these instances the wax used is generally lost, and there is always liability of the copper solution passing through, and causing what is termed surface deposit, making the face of the medal rough. We may remark that, although occasionally there may be a very good electrotype obtained from a plaster mould, still they are in general very inferior; as the saturating of the plaster has a tendency to blunt the impression, and the wax used for the purpose of saturation becomes expensive. It may be partially recovered by boiling the plaster in water: the wax melts out, and is obtained when the water cools Plaster should not be used for moulds where wax can be employed, being neither so good nor so economical; but there are cases in which the moulds being very large, the use of plaster is unavoidable.

MOULDS IN FUSIBLE ALLOY.-The next means of taking moulds is by fusible metal. This name is given to alloys of two or more metals which melt at a very low temperature; it suits the purpose of taking moulds of small objects very well. The following are examples of such compositions:

These all melt at a temperature below that of boiling water. The ingredients are melted together in an iron ladle, poured out upon a flat stone, broken up, and re-melted in the same way two or three times, in order that they may be thoroughly mixed. The medal from which the mould is to be taken is prepared in the same manner as described for wax.

The fusible alloy is melted and poured into a saucer, or, what