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of Sciences at Paris, in the spring of 1840, objects gilded, and which, after having been submitted to decisive proofs, have been acknowledged as nothing inferior in any respect, either in solidity or brilliance, to the best gilding produced by the mercurial method. These gentlemen have brought to the process which I have described some perfections which they have not made known. This mode of gilding appears to meet with some obstacles in its application to pieces of brass for the movements of clocks or watches, because the colour which it gives them is not such as is usually given to this species of work.

M. Droin, of Geneva, a distinguished workman, who is employed with M. M. Baute and Co., has succeeded, by following to the letter the process which I have described, in producing beautiful specimens of gilding ; but he has remarked, that in order to succeed it is necessary to have the metal that we want to gild (silver or brass) perfectly homogeneous, a quality which it is difficult at all times to meet with. The gilding will not be perfectly uniform except under this condition.

M. Perrot, of Rouen, has sent to the Academy of Sciences of Paris many samples of different metals which he has gilded by voltaic currents, and which appear to have succeeded very well.

M. Arago likewise presented the Academy of Sciences, in its session of the 5th of May last, with the spring of a chronometer from the manufactory of M. Dent, gilded to great perfection by means of the galvanic process. He called to mind, on this occasion, that he had presented to the Academy a multitude of objects in metal, gilded by means of the same process by M. Perrot. M. Perrot had already at this time, also, springs to exhibit which were gilded in the same manner, and if they were not comprised in the number of objects which he sent, it was because he was awaiting the completion of an experiment in which he had undertaken to gild, at the same time, all the movements of a watch ; to gild them not only whilst in their place, but whilst performing their usual functions.

In Germany, a distinguished artist of Stuttgard, Mr. Reinecker, who is the author of a very remarkable process of waving or watering, has brought the process of gilding by the galvanic method to that point of perfection which, under the relation of solidity, leaves nothing to be desired. As to its beauty, it leaves far in the rear every other species of gilding. “ If we consider,” adds the Gazette of Stuttgard (from whence we draw these details), “ that in the process of gilding by galvanism there is no disengagement of those mercurial vapours so hurtful to the operator, that the quantity of gold employed is much less than in the ordinary gilding, and that the greater part of the technical portion of the work is so simple that it may be executed by children, this discovery in this branch of industry will then appear of such importance that we can but express our ardent desire to see it generally employed.”

M. Boettiger, in the Annalen der Chemie and der Pharmacie (vol. xxxv., p. 350), describes the efforts which he has made to gild and platinize the plates of copper in relief which have bee obtained by the process galvanoplastik. .

He tried, at first, to platinize them by the galvanic method, making use of a solution of chloride of platina very neuter and very weak. He has succeeded, but he has found that it was necessary to have six immersions in the chloride of platinum to produce a covering of platinum of very small thickness; and again, this covering had not a very good colour, but rather grey than white.

Having found, by succeeding experiments, that for gilding no salt is preferable to the double chloride of gold and sodium, Mr. B. endeavoured to employ, in platinizing, the double chloride of platinum and sodium. The experiments which he has thus made have perfectly succeeded. In most cases he did not find it necessary to give more than three immersions to re-cover with a sufficiently strong layer of platinum the largest surface of copper; the colour also appeared of a much purer white. There is one circumstance, however, to which it is always necessary to pay attention in platinizing copper and other metals, which is, strongly to rub the object with a small piece of linen, and to scour it immediately with chalk each time, without exception, that it is withdrawn from the solution of platinum. It is worthy of remark also, that whether in silvering, in gilding, or in platinizing, those objects which have the highest polish are those also which take the metallic layer with the greatest facility and beauty. Mr. Boettiger thinks, that in order to produce good gilding on copper, it is necessary to commence by covering it with a cuticle of platinum, which is easy, and of very little expense ; the gilding is far more beautiful and more durable than that obtained by the direct gilding of the copper. It is easy to obtain the double chloride of platinum and sodium by mixing equal parts of dry chloride of platinum and of pure common salt in distilled water. It is very necessary also to rub the metal conveniently with fine sand, moistened hydrochloric acid mixed with chalk, in such a manner as to leave no traces of the oxyde of copper, for platinum will not be precipitated on those places where any of it remains, and it will suffice to rub again those places, in order to determine this deposit of platinum, when the plate of copper is again put into the solution as the negative metal of the pair.

I cannot avoid remarking, that I have urgently insisted, in my notice, on the necessity of well cleansing the surface of the copper from verdigris before putting them into the solution of gold, and on the importance of successive immersions, and of the rubbing being repeated after each immersion with a fine linen cloth.

M. Snaer, in a work on electro-metalurgy, of which we shall give an account in our next number, employs for gilding voltaic currents far more powerful than those which I have indicated, and

which have been generally used. He is said thus to obtain gilding much more solid, and of any thickness, as great as may be

required. He remarks that the process called the English one, of Elphinstone, does not offer this last advantage. In this process the object to be gilded, which is brass, for example, is put into a solution of double chloride of gold and sodium, raised to high temperature. The gold is precipitated by the effect of the solution of a part of the metal equivalent to that on which the precipitation takes place. Now, as soon as all the surface is re-covered by a thin cuticle of gold, there is no longer a possibility of any part of this surface being dissolved, and thus the gold is no longer precipitated. It can only form, then, a very superficial gilding, and consequently of short duration ; whilst by the voltaic process there is no limit to the thickness which may be given to the lamina of gold. Another inconvenience of the English process is, that there ensues, by the dissolution of a part of the surface to be gilded, an alteration of that surface, a circumstance which proves prejudicial in many cases, especially when it acts on objects the dimensions of which have been very exactly calculated, and ought not be altered, such as the wheels of chronometers, for example.

M. Hammon, an engraver at Geneva, has found great advantage in replacing the engraving by aquafortis, the varnish of which is made to cover the plate of copper by a cuticle of gold precipitated by voltaic agency: the tracings of the needle are far more delicate. The lamina of gold being permanent, and not being broken or carried off like the coating of varnish, when the aquafortis has operated, we may with great facility correct the engraven plate if it has any defect, which is impossible in the old process. We shall give to our readers, in one of our next numbers, a specimen of this species of engraving, by means of which we have produced a new apparatus which we purpose describing on a future occasion.

ROYAL VICTORIA GALLERY, MANCHESTER.

CONVERSAZIONE, FEBRUARY 16, 1841. On a New Class of Magnetic Forces. By J. P. Joule, Esq.

As it is my intention to bring forward in this paper an electromagnetic principle, in reference partly to the motion of machines, I hope that a few preliminary observations with respect to the present class of electro-magnetic engines will not be deemed out of place.

The great attractive powers of the electro-magnet, and the extreme rapidity with which its polarity is reversed by changing the direction of the current, very readily presents to the reflecting mind an idea that its power may be made available for mechanical purposes. Accordingly, as soon as the general principles of electromagnetism were understood, Professor Henry, Mr. Sturgeon, and

after them a great number of ingenious individuals, constructed various arrangements of machinery to be set in motion by magnetic attraction and repulsion.

At that period the expectations that electro-magnetism would ultimately supersede steam, as a motive force, were very sanguine. There seemed to be nothing to prevent an enormous velocity of rotation, and consequently an enormous power, except the resistance of the air which it was easy to remove, the resistance of iron to the induction of magnetism which I succeeded in overcoming to a great extent by annealing the iron bars very well, and the inertia of the electric fluid.

We are indebted to Professor Jacobi for the exposition of the principal obstacle to the perfection of the electro-magnetic engine. He has shewn that the electric action produced by the motion of the bars operates against the battery current, and in this way reduces the magnetism of the bars, until, at a certain velocity, the forces of attraction become equivalent to the load on the axle, and the motion in consequence ceases to accelerate. The Russian philosopher had not, however, given precise numerical details concerning the duty of his apparatus, nor had he then determined the laws of the engine; I was therefore induced to construct an engine adapted for experimental purposes, in order in some measure to clear up

these points. The experiments which I instituted two years ago with this instrument, were fully detailed in the Annals of Electricity, vol. 4, p. 474 ; I need not, therefore, allude to them in this place further than by stating some of their principal results.

I found, 1st,—That the counter electric action, or, in other words, the magnetic electric resistance to the battery current, is proportional to the velocity of rotation and the magnetism of the bars.

2nd,—That the economical duty at a given velocity of rotation is a constant quantity, whatever the number of similar pairs in series (provided that the resistance of the battery is kept the same).

3rd,—That at small velocities great advantage is obtained by reducing as far as possible the resistance of the battery, and by arranging the coils so as to facilitate as far as possible the transmission of the current.

And 4th,—That the economical duty at a given velocity, and for a given resistance of the battery, is proportional to the mean intensity of the several pairs of the battery.

With my apparatus, every pound of zinc consumed in Grove's battery produced a mechanical force (friction included), equal to raise a weight of 331,400lbs. to the height of one foot, when the revolving magnets were moving at the velocity of eight feet per second.

Now the duty of the best Cornish steam-engines is about 1,500,000lbs. raised to the height of one foot by the combustion of each pound of coal, or nearly five times the extreme duty that I was able to obtain from the magnetic engine by the consumption of

a pound of zinc. This comparison is so very unfavourable, that I must confess that I almost despair of the success of electro-magnetic attractions as an economical source of power ; for although my machine was by no means perfect, I do not see how the

arrangement of its parts could be improved so far as to make the duty per pound of zinc much superior to the duty of the best steam-engines per lb. of coal. And even if this were attained, the expense of the zinc and exciting fluids of the battery is so great, when compared with the price of coal, as to prevent this class of magnetic engine from being useful for any but very peculiar purposes.

NEW CLASS OF MAGNETIC FORCES.

A few weeks ago an ingenious gentleman, of this town, suggested to me a novel form of electro-magnetic engine. He was of opinion that a bar was increased in length by receiving the magnetic influence; and that, although the increment was perhaps very small, it still might be found valuable as a source of power on account of the great force with which it would operate. At that gentleman's request, I have entered into experiments to ascertain whether his opinion was correct, and if so, into a calculation whether the new source of power might be advantageously adopted for the movement of machinery

After some preliminary trials, I adopted the following method of experiment. A length of thirty feet of copper wire, one twentieth of an inch thick, and covered with cotton thread, was formed into a coil twenty-two inches long and one-third of an inch in interior diameter. This coil was secured in a perpendicular position, and the rod of iron, of which I wished to ascertain the increment, was suspended in its axis so as to receive the magnetic influence whenever a current of electricity was passed through the coil. Lastly, the upper extremity of the rod was fixed, and the lower extremity was attached to a system of levers which multiplied its motion three thousand times.

A bar of rectangular iron wire, two feet long, one-quarter of an inch broad, and one-eighth of an inch thick, caused the index of the multiplying apparatus to spring from its position, and vibrate about a point, one-tenth of an inch, in advance, when the coil was made to complete the circuit of a battery capable of magnetizing the iron to saturation, or nearly so. After a short interval of time the index ceased to vibrate, and began to advance very gradually in consequence of the expansion of the bar from the heat which was radiated from the coil. On breaking the circuit, the index immediately began to vibrate about a point, exactly one-tenth of an inch, lower than that to which it had attained.

By multiplying the advance of the index by the power of the levers

, we have godor of an inch, the increment of the bar, which may be otherwise stated as yooooo of its whole length.

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