rituous liquors, and when the product resulting from it is well rectified, and marks from 36 to 60 degrees of the aërometer of Beaumé, then I do not hesitate to advance, that in this state it is impossible to discover from what kind of alcohol this ether is prepared: it seems, at the instant of the effusion of the acid on the spirituous fluid, that its characteristic aroma vaporises or is destroyed by the disassociation or alteration of the elements that compose it *.

I shall add, that the different sorts of alcohol, at an equal degree of rectification, do not always produce the same quantity of ether. I have met with some kinds that have yielded one-sixteenth more than others. Is this owing to the heterogeneous principles which they contain, more or less, or to the various proportions of the elementary substances from which they are composed, or to a more considerable quantity of aroma, which I believe to be inherent in every sort of spirituous liquor?

Without seeking now to explain these variations, I shall only notice, that the alcohols which appear to yield the most ether, are those extracted from perry, wine, and cider; those from Kircheuassor, rum, gin, and brandy made from grain, give a much inferior quantity.

By the effect of the re-action of the concentrated acid on the alcohol, and by the aroma that proceeds from it, one may always distinguish the nature of the ardent spirit employed in this operation; but as soon as the mixture is half cold, then the liquor that results is perfectly identical, as to its smell only, but the colour is more or less intense: this appears to depend on the different proportions of the oleaginous aromatic principle, which characterises each kind of alcohol, and on which the acid particularly acts by carbonising it, &c. &c.

VOL. XXV.-SECOND SERIES.

I i

On

On the Composition for tinning the Backs of Mirrors.

From the BULLETIN DE LA SOCIETE ENCOURAGEMENT.

THE

HE Committee of the Chemical Arts of the Society of Encouragement have been charged to examine the mixture employed by M. Verea, for the tinning of looking-glasses, and to find out its proportions, and thus to complete the remarks transmitted by the author to the Society. The Committee has besides entered into some details on the application of this process, and has presented to the Society a particular report on this object.

M. Verea made three experiments in the presence of the Committee; he employed each time a different mixture; twice he brought with him what he had occasion to use; but at the third sitting, some tin and pure lead were given to him, and of which he at length used an ingot, containing 61 per cent. of tin and 39 of lead.

The metallic leaves that were obtained from this mixture were much better than what he had made with the metal which he brought for the second experiment, but not near so good as those that were produced from the mixture that he brought when he first came to the Committee.

After each experiment a seal was fixed upon a piece of the mixture employed. The ingot, No. 1, being the mixture which had produced the finest sheets or plates, and the best tinning on the glass, was that of which it was most important to ascertain the composition. It was analysed with all possible care; and the following results were obtained. This mixture much resembles the solder used by plumbers, it has the appearance and fracture of common tin vessels; its surface is smooth, and without

any

1

any appearance of crystallisation. Its specific weight is 84,221; it cuts kindly, is very ductile, and rolls easily. into very thin sheets. The analysis, twice repeated, has proved that it contains, per cent, 65 of tin, 35 of lead. To verify the results given by the analysis, 650 grs. of tin and 350 grs. of lead were melted together, and with this mixture some metallic plates were made, as good in all respects as those which M. Verea bad presented to the Society.

This then is the nature of M. Verea's mixture. There is no difficulty in its preparation, since it is only necessary to melt in an iron ladle or a cast-iron boiler 65 parts of fine tin, and to add 35 parts of pure lead, to stir the mixture well together, to run it into an ingot, and to leave it to cool in the same vessel, at least when it is not used directly.

I. On tinning Mirrors, and on the Fabrication of the metallic Sheets or Plates made by M. Verea's Process. The process employed by M. Verea, whether for tinning mirrors or making metallic plates with one surface perfectly polished, is as follows.

It is begun by fixing upon the glass intended to be tinned, or with which the metallic plates are to be made, a piece of cork or wood, glued to the centre, on the least polished side, so as to form a handle, by which it may be held. The glue being dry, and the cork firmly fixed, the mixture having been previously prepared, is melted over a slow fire, in a flat vessel, of about double the surface of the glass that is to be tinned.

When the metal is well melted it is slightly agitated, in order to render the mixture homogeneous; the surface is skimmed by drawing a card or strip of thick paper over it; the surface of the glass, to which the metal is

Ii 2

to

to be applied, is moistened by gently breathing upon it; it is carefully wiped, in order to clean it; it is slightly heated, in order that it may not break at the moment it comes in contact with the melted metal; it is again. wiped; and when the mixture begins to thicken on the edges, the card or strip of paper, which is held in the left hand, is passed over the surface of the melted metal; the plate of glass is immediately applied to it, and then detached by raising it, not vertically, but in a curved direction from right to left, of which the centre of motion is the top of the piece of cork which serves as a handle; a slight shock is given to it, in order to shake off the melted metal that adheres to the lower extremity of the metallic plate, and which without this precaution would be of an unequal thickness.

If the operation has been well performed, the bed of metal that is applied to the glass, and which cools and becomes solid in the same moment, will adhere to it so as not to separate from it when the glass is raised from the surface of the melted metal.

We see that if the metal be too warm it cannot form a solid metallic pellicle on the surface of the glass: that the same difficulty occurs if the glass be too hot, or if it be left too long upon the melted mixture. The metallic plates, on the other hand, are too thick when the metal is too cold, or when the glass has not been made sufficiently warm. They are tarnished and covered with streaks when the metal has not been well skimmed, also when the glass has not been instantaneously applied, and with all the necessary care, or when the mixture contains ei ther copper or iron, because these metals give it the property of crystallising with facility while cooling.

It would be useless to enter into more particular details on the practice of this process, because habit alone

is necessary to be enabled to give to the metallic plates made by his means the suitable degree of brilliancy and thickness, and this habit is easily and cheaply acquired, since a great number of trials can be made in succession, and the faulty plates being re-melted, no sensible waste is incurred.

The metallic plates prepared in this manner adhere to the glass when cooled with so much strength, that it requires a certain degree of force to separate them. A point or the blade of a sharp knife must be passed between the edge of the glass and the metal; and a little air thus introduced between the two surfaces, renders it easy to separate the metallic sheet from the glass. This sheet or plate has the polish of the glass on the side that was in contact with it; the opposite surface, on the contrary, is unequal, rough, and covered with crystals, which are larger and more numerous according as the tin and lead employed in the preparation are more or less pure. Verea operates thus: If the contact of the metallic plate with the glass be not so exact or perfect as it ought, and the image of the objects reflected by it not so clear, he is obliged to glue a strip of paper on the edges of the glass, in order to prevent the air, and especially any dirt and moisture, from entering between the glass and the metal, which without this precaution would be liable to oxydate and lose all its brilliancy.

M.

When the metallic plates are to be prepared, the same glass may always be used, as long as the plates are required to have the same surface, only the glass must be carefully suffered to cool when it becomes too warm to give the plates the right thickness; and this may be easily done during the time that the mixture is being warmed when it becomes too cool. If it is desired to make metallic plates by this process of more than 400 or

450