other modes which I have described for cleaning the plate, will answer as well.

(128.) The adhesion, or buttoning of one metallic plate to another, must not be confounded with apparent adhesions of the duplicate to the original, arising from the copper growing round the edge, and firmly embracing it. This is to be remedied in a great measure, in the first instance, by coating the edge with a layer of lac varnish or grease, which prevents the deposit taking place at that part. After a considerable lapse of time, the plate increases laterally, and covers the coating.

(129.) The lateral growth of a plate is a property of considerable importance, for if a particle of non-conducting substance be placed upon a metal, it will be covered. In this way, drawings made on copper, with varnishes, may be multiplied. If a non-conducting substance is to be copied, by means of a thin film of conducting substance, a break in the continuity of the latter will not prevent the formation of a perfect plate. For the same reasons, care must be taken that no air or gas-bubbles adhere to the plate, for in like manner they will be infilmed, and leave a little flaw or gap in the duplicate plate. To cast metals upon an air bubble, seems at first too wonderful to be believed, and in former times, would doubtless have subjected the discoverer to destruction, upon the supposition that he was in communication with an evil spirit; but in these latter days, we find that it is even more difficult to prevent than to effect.

CHAPTER II.

ON SUBSTANCES CAPABLE OF RECEIVING THE METALLIC DEPOSIT.

Substances on which the deposit may take place, 130-131. Metals, 132–136. Non-conducting substances; Sealing Wax, White Wax, 136-139. Absorbent substances, as Paper and Plaster of Paris, 139–141. Means of rendering them non-absorbent, 141-142. Means of copying non-conducting substances, by Metals, by Plumbago, 143–145. Comparison between the methods, 145.

(130.) The voltaic deposit of metal may take place upon any conducting substance, which is capable of acting the part of the negative metal, in the arrangement. The laws which relate to this, are the same which regulate, in a similar manner, the plates of the battery. The deposit may be effected upon most metals, except the earthy and alkaline, and upon any alloy or compound of them. It may likewise take place upon charcoal and plumbago. When the metals are employed, the effect is evident enough, for the arrangement differs in nothing from that of a Daniell's battery.

(131.) Where we desire the duplicate to possess a surface and form exactly like those of the original, it is of the utmost importance that the metal on which deposit is to take place, should not of itself decompose the fluid, because, in that case, the duplicate is sure to be more or less impaired. To illustrate this, zinc, lead, tin, or iron, in sulphate of copper, precipitates the copper immediately from its solution, but the former metals are dissolved, exactly in equivalent proportion with the reduction of the latter. The solution of this metal impairs the surface, and renders the duplicate less perfect. This may be prevented, in a great measure, by taking care that the voltaic current is passing at the moment when the metal is plunged into the fluid; and this mode of proceeding is supposed, by many, entirely to supersede the elective affinity, as it is termed, or the spontaneous action of the metal on the fluid. But I

can decidedly affirm, that a battery of twelve cells will not entirely prevent the solution of the more oxydable, and the reduction of the less oxydable metals.

(132.) The metals which can be employed with advantage to receive a deposit of any other metal, are therefore those which are not acted upon by the particular fluid in which they are immersed; those however, which are but slightly acted upon, may still be employed. Platinum, from its being unaltered by any solution, holds an important place for the reception of every metal; its great value however, must ever be an impediment to its general use.

(133.) Gold is equally valuable with platinum, but is still more expensive; yet when extended to that state in which it exists as gold leaf, it may be applied over the surface of any soft substance, and thus a metallic surface is presented. This mode may be employed with other metals, such as silver or tin; but we have other methods, which render all these modes useless.

(134.) Silver is only reduced by gold and platinum, and therefore may be employed for the reduction of metals, when we require the deposit to be of very pure metal. Silver leaf of a thickness of about one square foot to the ounce, and made of pure metal, is much used by the forgers. The process they adopt is, to place the coin to be copied on a piece of wood, and upon the coin they place a piece of this thin silver. They beat it gently with a wooden mallet, till a perfect impression is taken on the metal, a result soon obtained. They then copy the opposite side of the coin in the same way. The two impressions are then soldered together, and the manufacturer sallies forth and risks his neck for the illicit shilling, which has cost him this labour. The reader will doubtless have no inclination to practise this fraud, and therefore it is unnecessary to enter into the process farther; but it should be borne in mind, that the same means may be employed, with a better intention by the electro metallurgist, to obtain a mould.

(135.) The alloys of lead which are principally employed, are pewter, the fusible metal, and the type metal. The first is an alloy, consisting of about eighty parts of tin, and twenty of lead, but for many purposes, more lead might be added. There are many varieties of this alloy, containing either copper, antimony, or bismuth, but the first I believe will be found to be the best. The fusible metal has been much used by the electro-metallurgist for small casts, but its manipulation is difficult. It should be melted over a lamp, and the surface skimmed perfectly clean. A portion is then to be poured upon any flat surface, and the medal is to be placed upon it with a jerk, and firmly pressed. The metal should be nearly at the point of congealing before the impress is given, or the surface of the cast is apt to exhibit a crystalline appearance. The fusible metal of Sir Isaac Newton, contains lead, bismuth and tin, but mercury is generally added by the instrument makers to render it more easily fusible; the mercury, however, should always be omitted, when the alloy is to be used for taking casts. The composition of the type metal is stated to be about one part of lead, to sixteen of antimony, with a small portion of copper. Considerable practice is required to make casts either in the fusible or type metal, and I am informed that even in type foundries, a man rarely excels in the casting of more than a few letters.

The Italians have a method of taking very perfect moulds with pewter. They take a portion of the melted pewter and place it on a piece of paper; upon this they lay the medal, and under both a piece of carpet; upon the medal they place a log of wood, and then a sharp blow on the wood will ensure the sharpness of the cast. The worth of a cast thus made, is from six pence to half a crown.

An impression may be given to a perfectly clean bright surface of sheet lead, by placing upon it the object to be copied, and then with a steady hand dealing a heavy blow. By this mode even a sealing-wax impression may be copied, although this at first sight would appear hardly credible. By

pressure alone, it would be difficult to obtain the result, which can be given by the blow.

Rolled lead, first scraped, in order to remove any oxide from the surface, and then flattened by running it through a press upon a polished iron plate, will readily take the impression of the most delicate work or engraving. The object to be copied, is simply to be placed upon the lead, and then the two are to be sent once, and once only, through the printing press, as in the ordinary operation for taking a print. This mode is perfect, and answers well for any case. The pressure in rolling is far greater than can be given by direct pressure, though there are instruments, which are used by embossers, capable of exerting great power.

The metal employed for stereotyping, is applicable to the electrotype.. A mould in this metal is taken in a particular way from plaster, but it requires certain apparatus which there is no need to describe, as any article can be easily obtained from the foundry.

(136.) Non-conducting substances are of three kinds; substances having no affinity either for the metal or the solution; substances acted upon by the solution; and lastly, substances capable of combining with the metal thrown down. Those of the first class are by far the most valuable, but are not The best of these is sealing-wax, a compovery numerous. sition of shel-lac, venice turpentine, and colouring matter. Dr. Ure gives, as the proportion in which these are used, four, one, and three. The manufacturers have several varieties, the most expensive of which is the best for making seals. Some of them are extremely hard, as for example, a black wax which is used for filling up the letters in the engraved plates of shop windows, but I do not know how a difference of composition can affect the properties of the wax in this important manner. The use of sealing wax is attended with considerable expense, as good wax cannot be purchased under three and sixpence or four shillings a pound, but it takes impressions of objects of the greatest delicacy with the