solution of bi-carbonate of potash, for during this prolonged ebullition the perchloride passes slowly, and with difficulty it is true, to the minimum. The liquor also takes a greenish hue; but the choice of the bi-carbonate of potash has a great influence on the result. This salt contains almost always some traces of organic substances capable of reducing the perchloride of gold to the state of proto-chloride. When the bi-carbonate of potash is too palewhen these organic matters are wanting, the operation is carried on with difficulty; whilst the presence of these same matters renders it very easy to conduct. Besides this, sulphurous acid, oxalic acid, the salt of sorrel, and many other organic matters or minerals, may be used to produce this effect, and there is nothing to prevent them from adding to the liquid, little by little, until the complete return of the gold to the inferior state of chloridation.

After these proper trials, your Commission is disposed to believe that the opinions of M. M. Wright and Elkington are established. It regards, then, the liquid employed in gilding by the humid process, as essentially formed of a combination of proto-chloride of gold and of chloride of potassium dissolved in a liquid, highly charged with carbonate and even the bi-carbonate of potash, which may be well understood if we consider the liquid as containing the prot-oxyde of gold dissolved in the potash, and supposing all the chloride to be in the state of chloride of potassium.

If experience should shew in future that the metals are precipitated better when we take their solutions in the same state of saturation as that of the salt which ought to replace them, the remark of M. M. Wright and Elkington will be of importance. They think, in fact, that the cause which ensures the success of the gilding by the humid method is, that the chloride of copper which is produced being a chloride of 2 atoms chlorine, we ought to employ a chloride of gold, also containing 2 atoms of chlorine, and not a chloride which contains a chlorine of 3, as is the case for the per-chloride of gold.

In conclusion, in order to appreciate the true character of the gilding by the humid method in the arts, it will suffice us to report here the analysis of different plates gilded, either by mercury or by the humid process, and proved, by the pains of our colleague, M. d'Arcet, at the laboratory of the mint. Plates formed of mixed metals, known in commerce under the name of bronze, were sent to different manufacturers, with orders to have them gilded. They endeavoured to obtain the strongest gilding, and that of the weakest description, still keeping, in all instances, within the limits of the usages of commerce.

The following are the results obtained from plates of 1 decimetre square :

Quantity of Gold by the Square Decimetre in the Gilding by

Mercury.

By M. Plu. By M. Denière. By M. Beaupray.

The quantity of gold in the two cases varied, then, in the relation of 100:16.5, or sensibly as 6:1.

We will, however, now give the results obtained by the humid method:

Quantity of Gold by the Square Decimetre in Gilding by the

Humid Method.

By M. M. Bonnet and Villerme. By M. Elombert.

Thus, the best gilding by the humid method having fixed 0.0422 of gold to the square decimetre, and the poorest gilding by the mercurial method having taken 0.0428, we see that the gilding by the humid method scarcely attains, in the most favourable case, to the degree of thickness which the worst gilding with mercury is obliged to attain.

These are, then, two distinct branches of industry; the one cannot replace the other.

2nd. The Galvanic Process of M. Elkington.

As this process is so very simple, and as its description is not very long, we will likewise give the text of the patent; here an analysis will suffice.

M. Elkington takes 31 gramme 25 centigrammes of gold, converted into oxyde, 5 hectogr. of prussiate of potash, and 4 litres of water. These must be boiled together for half an hour; at that time the liquid is ready to use. Boiling, it gilds very quick; cold, it gilds more slowly. In both cases, we immerse therein the two poles of a pile of a constant current, the object to be gilded being suspended at the negative pole, where the metal of the solution will be deposited.

In the patent of M. Elkington, the word prussiate of potash, which is employed without any other definition, leaves us in uncertainty, for chemists are acquainted with three prussiates of potash : the simple prussiate, the yellow ferruginous prussiate, and the red prussiate. The agent of M. Elkington, when pressed to explain this point, informed us that the patent intended the simple prussiate of

the cyanuret of potassium to be understood. In fact, when he submitted his process before us, it was the simple cyanuret of potassium that was made use of.

In the trials that we have made of the process of M. Elkington, we have gilded brass, copper, and silver.

In operating on a silver dessert spoon, with the liquid raised to 60° centigrades, we obtained a rapid and regular gilding. Scarcely was the spoon immersed ere it became converted into gold. During each minute there was deposited about 5 centigrammes, and we have not prolonged the experiments, but after six successive weighings we have found that the quantity remained the same for the same time.

We can, then, augment the thickness of the lamina of gold at pleasure, and calculate this thickness by the duration of the immersion.

But the simple cyanuret of potassium is an expensive salt, difficult to preserve in solution, to the employment of which will be raised divers obstacles in the manufacture, and it remains doubtful whether, in employing it, the gilding is performed to better account than by the actual method of mercury.

3rdly.-Galvanic Processes of M. DE RUOLZ, for the Application of a great number of Metals on other Metals.

As we have taken occasion to remark, previously, whilst M. Elkington solicited an addition to his patents, M. de Ruolz, on his part, took out a patent of invention for the same object. The patent of improvement of M. Elkington is dated Dec. 8th, 1840, that of M. de Ruolz is dated 19th December. Everything shews that M. de Ruolz has proceeded on his side without knowing the demand of M. Elkington; besides his processes are still very different to those of the English artist.

Leaving aside these questions of patent, which it is not our province to examine, and confining ourselves to the scientific discussion, we proceed to lay before the Academy the remarkable results obtained by M. de Ruolz.

Gilding. In order to apply gold, M. de Ruolz employs the pile, like M. M. de la Rive and Elkington, but he has submitted to experiment such a variety of solutions of gold, that he has rendered it easy to find cheaper and more suitable ones than those made use of by M. Elkington himself.

Thus he made use of 1st, cyanuret of gold dissolved in the simple cyanuret of potassium; 2nd, of cyanuret of gold dissolved in yellow ferro-cyanuret; 3rd, of cyanuret of gold dissolved in the red ferro-cyanuret; 4th, of chloride of gold dissolved in the same cyanuret; 5th, of the double chloride of gold and of potassium dissolved in the cyanuret of potasium; 6th, of the double chloride

*

of gold and of sodium dissolved in soda; 7th, of the sulphate of gold dissolved in the neuter sulphate of potassium.

Even chemists will be surprised to hear of these processes, of which the last-named-that which depends on the employment of the sulphurets is the most convenient, and which, applied to gild the metals, such as bronze and brass, the sensibility of which is known in regard to sulfuration, it succeeds to the marvel, giving the most beautiful gilding, and the purest of tone.

The remaining processes succeed well, and the three latter, in particular, are applicable to all the metals used in commerce, and even on those metals which hitherto have not been employed in this manner.

Thus we can gild platinum, either on its whole surface, or on certain parts, so as to obtain designs in gold on a platinum ground.

Silver becomes gilded with great facility, so regularly, and with colours so pure and so beautiful, that the hope is permitted that at a future day all the vermillion will be obtained in this manner. The thickness of the lamina of gold may be varied at pleasure, and even its colour. A mixture of mat and polished gold may be obtained on the same piece. In fine, we may gild with equal facility objects of large dimensions, in plates or in relief, hollow or engraven, on wires of the thinnest and finest qualities. Of the samples laid before the Academy, we refrain from giving a detailed description in this regard.

All that has just been said in regard to silver, may be applied to copper, brass, or bronze. Nothing can be easier or more regular than the gilding of objects of different natures, which are fabricated in commerce with these three metals. Whilst gold, applied in pellicles excessively thin, constitutes a varnish sufficient to preserve them from oxydation; whilst, applied in thicker laminæ, it is capable of resisting, besides, all friction in its use; by a very simple artifice, the thickness of the layer of gold may be varied, so as to leave it thin in those places where the action of the air alone is feared, and to thicken it, on the contrary, where it is of importance to guard against the waste caused by friction against the parts. The jewellers will derive great advantage by these means, and science also will find her part in these advantages. Thus there is nothing to hinder us, in future, to gild, with little expense, all those instruments of copper which consume so rapidly in our laboratories, to procure us our tubes, capsules, and crucibles, of copper gilt, which will replace the vases of gold which are sometimes necessary, and which no chemist does not possess at the present day.

In fact, amongst the pieces deposited on the bureau of the Academy, there was a capsule of gilded copper, which had resisted very efficaciously the action of boiling nitric acid.

Packfong takes very well the gilding by this process, and it becomes

* The analogous salt of potash did not succeed.

easy to convert into vermillion the enamel in packfong, already scattered, and which are not without danger.

Steel and iron are gilded well and solidly by this method, though not in any relation, in this respect, with the old and very imperfect processes of gilding on iron or steel: it is only necessary to commence by putting on the iron or steel a cupreous pellicle.

Dessert knives, the instruments for the laboratory, the instruments of chirurgery, arms, the frames of spectacles, and numerous other objects in steel or iron, will receive this golden varnish with facility and economy. We have proved that divers objects of this nature have been received with lively satisfaction by the commercial world. The employment of gilded knives to common usage has caused us to observe, besides, that this application was of a nature to insist wearing, after being long in use, when the lamina of gold was only of a slight degree of thickness.

Pewter, under this relation, has been the object of the very interesting experiments of M. de Ruolz. He is convinced that it does not gild very well by itself; but when it comes to be covered with an infinitely thin pellicle of copper, by means of the pile and a cupreous solution, it becomes as easy to gild as silver. The silvergilt pewter itself is of such a beautiful description, that it is certain that the commercial world well knows where to find profitable openings for this new product; it, however, becomes our duty to add, that by reason of the very high price of gold, it is difficult to put upon these covers of pewter a stratum of gold sufficient to render them durable, without too much raising the price of them.

The Commission has taken great pains to enlighten itself in a precise manner on these circumstances of the operation, by means of which gold is applied on the different metals. Different questions presented themselves. Can we, in fact, augment at will the thickness of the layer of gold, in such a manner as to produce the same effects as by the agency of mercury, or even in such a manner as to go further? Is the deposit of the metal made regularly, or in a variable manner? What was the part taken by the temperature of the liquid, of its concentration, of the number of elements of the pile, and the nature of the metals employed. Your Commission, without pretending to search into these questions, as they will be the subject of lengthened researches, are desirous, at present, to confine themselves plainly to treat them in a practical point of view.

1st. The precipitation of gold is regular; it is exactly proportionate to the time of immersion, a valuable circumstance, which permits us to judge of the thickness of the gilding by the duration of the operation, and to vary it at pleasure. In order to prove this, it will suffice to report here some of our experiments.

We operated on a liquid, containing 1 gramme of dry chloride of gold dissolved in 100 grammes of water, containing 10 grammes of yellow ferro-cyanuret of potassium.