of wood are, take a pair of pincers or tongs, to retain the substance to be examined under water.

First weigh the body in air; then having balanced the tongs in water, fix to it the body to be weighed, which being lighter than water, will raise the tongs, and cause the other scale to preponderate. Observe the loss of weight of the body in water, and proceed as before. There are some things that cannot be weighed in this manner, such as quicksilver, fragments of diamonds, &c.; these must be put into a glass bucket, hanging to the scale. For finding the specific gravities of fluids, a solid glass bubble is used.

CHAPTER VIII.

MANUFACTURES.

SECTION I.

MANUFACTURE OF TIN PLATE.

261. THIS art was introduced into England from Bohemia, about the year 1670.

Bar iron of the finest quality, called tin-iron, made with the greatest care for this particular purpose, is first cut to the necessary length, and then rolled in a mill, into plates of the requisite thinness. These plates are then cut by hand-shears to the sizes suitable to the different markets. As the shearer shears the plates, he piles them in heaps, occasionally putting one plate the cross way, to keep each box separate:-Two hundred and twentyfive plates being called a box.

The iron plates now go into the hands of the scaler, who bends each of them singly across the middle, into this form (A) preparatory to their being

cleaned for tinning, and for the conveniency of putting them into the scaling furnace.

This furnace, or oven, is heated by flame thrown into it from a fire-place of a peculiar construction; the plates are put into the oven in rows, and arranged three in each row, until the oven is full. It will be obvious, that if they lay flat on the floor of the oven, the flame could play only on one side of each plate; whereas, by being in a bent form, the Aame can operate equally on both sides.

The operation of cleansing, which is preparatory to the process of scaling, is commenced by steeping the plates for the space of four or five minutes, in a mixture of muriatic acid and water, in the proportion of four pounds to three gallons. This quantity of the diluted acid will generally be sufficient for 1800 plates, or eight boxes of 225 plates each.

When the plates have been steeped for the time prescribed, they are taken out of the liquor, and placed upon the floor, three in a row, and then by means of an iron rod put under them, they are conveyed to a furnace heated red-hot, where they remain until the heat takes off the scale, the removal of which was the object in submitting them to that high temperature.

When this is effected, the plates are taken to a floor, where they are suffered to cool;-they are then straightened, and beaten smooth upon a cast-iron block. If the rust or oxyde which was attached to the iron, has been properly removed, they will appear mottled with blue and white, something like marbled paper.

They are now rolled a second time between a pair of castiron rollers, properly hardened and finely polished. This operation makes both sides of the plates perfectly smooth, and imparts a sort of polish to their surfaces. These rollers are each about 17 inches long, and 12 or 13 inches in diameter.

These rollers are used without heat, but they are screwed down very close one upon the other, only leaving bare room for the plates to pass, that the utmost attainable degree of pressure may be given to them.

The plates are now put one by one into troughs filled with a liquid preparation called the lies. This is merely water, in which bran has been steeped for nine or ten days, until it has acquired a sufficient acidity for the purpose. The design

of putting the plates into the troughs singly, is, that there may be more certainty of the liquor getting between them, and both the sides of every plate being soaked alike in the lies. In this liquor they remain ten or twelve hours, standing on the edges; but they are turned, or inverted, once during that time.

The next operation is that of steeping in a mixture of sulphuric acid and water, in proportions which vary according to the judgment of the workmen.

The trough in which this operation is conducted, is made with thick lead; and the interior is divided by partitions of lead. Each of these divisions contain about one box of plates. In the diluted sulphuric acid which is in the different compartments of this vessel, the plates are agitated for about an hour, or until they have become perfectly bright, and entirely free from the black spots which are always upon them when they are first immersed in it.

Some nicety, however, is required in this operation, for if they remain too long in the acid, they will become stained. This and the former process with the acidulated water, are both hastened by giving to those menstrua an increase of temperature; and this is effected by means of heated flues which run underneath the trough.

When the plates come out of the pickle, they are put into pure water, and scoured in it, with hemp and sand, to remove any remaining oxyde, or rust of iron, that may be still attached to them, for wherever there is a particle of rust, or even dust upon them, there the tin will not fix. They are now put into fresh water to be there preserved for the process of tinning, for it has been found that they will acquire no rust, although they should be kept twelve months immersed in water.

All these processes are preparatory measures for the operation of tinning.

For this purpose an iron pot is nearly filled with a mixture of equal quantities of block and grain tin, in a melted state; and a quantity of tallow or grease sufficient, when melted, to cover the fluid metal to the thickness of four inches, is put into it.

When the tin-pot has been charged, the metal is heated from a fire-place underneath, and by flues which go round the pot, until it is as hot as it can be made without actually inflaming the grease which swims upon its surface. The use of the grease is to preserve the tin from the action of the

atmosphere, and to prevent it from oxydating. By melting a little tin or lead in an iron ladle, and, when the dross is skimmed off, putting a morsel of tallow upon the metallic fluid; the effect of the tallow in cleansing the face of the metal will be evident. The workmen say, that it also increases the affinity of the iron for the tin, or, as they express it, that it makes the iron-plates take the tin better.

Another pot, which is fixed by the side of the tin-pot, is filled with grease only, and in this the prepared plates are immersed, one by one, before they are treated with the tin; and when the pot is filled with them, they are suffered to remain in it as long as is necessary. If they remain in the grease an hour, they are found to tin better than when a shorter time is allowed them.

From this pot they are removed, with the grease still adhering to them, into the pot which contains the body of melted tin; and in this they are placed in a vertical position. Three hundred and forty plates are usually put into this pot at once; and, for the sake of their being thoroughly tinned, they usually remain in it one hour and an half.

When the plates have laid a sufficient time immersed in the melted tin, they are taken out and placed upon an iron grating, that the superfluous netal may drain from them; but notwithstanding this precaution, when they become cold there is always more metal found adhering to them than is necessary; and this is taken off by a process called washing.

In the first place, the wash-man prepares an iron pot, which he nearly fills with the best grain-tin in a melted state; another pot of clean melted tallow or lard free from salta third pot with nothing within it but a grating to receive the plates-and a fourth, called the listing-pot, with a little melted tin in it, about enough to cover the bottom to the depth of a quarter of an inch.

The pots being all in a state of fitness, the wash-man commences by putting the plates which have undergone the various operations hitherto described, into the vessel of graintin called the wash-pot. The heat of this large body of melted metal, soon melts all the loose tin on the surface of these plates, and so deteriorates the quality of the whole mass, that it is usual, when 60 or 70 boxes have been washed in the grain tin, to take out 300 weight, and replenish the washpot with a fresh block of pure grain tin. These vessels generally hold three blocks each, or about half a ton weight of metal.

When the plates are taken out of the wash pot, they are carefully brushed on each side with a brush of hemp of a peculiar kind, made expressly for the purpose.

1

The wash-man now takes a few plates out of the wash-pot, he then takes one plate up with a pair of tongs, which he holds in his left hand, and with a brush held in his right hand brushes one side of the plate-he then turns it, and brushes the other side, and immediately dips it once more into the hot fluid metal in the wash-pot, and without letting it out of the tongs, instantly withdraws it again, and plunges it into the grease pot.

Practice gives the workman so much expedition, that an expert wash-man, if he makes the best use of his time, will wash 25 boxes, consisting of 5,625 plates in twelve hours; notwithstanding, that every plate must be brushed on both sides, and dipped twice into the pot of melted tin.

The use of the grease pot is to take off any superfluous metal that may be upon the plates ;-but this is an operation that requires great attention, because, as the plate is immersed in the grease while the tin is in a melting state upon it, a part must run off, and the remainder become less and less while the plate continues in it; therefore, if these plates should ever be left in the melted tallow longer than is absolutely necessary, they will doubtless require to be dipped a third time in the tin. On the other hand, if the plates were to be finished without passing through the grease, they would retain too much of the tin, which would be a loss to the manufacturer, and besides, the whole of the tin would appear to be in waves upon the iron.

The plates are now sorted, smoothed, and packed in boxes of 225 each, for sale; and of these are manufactured much of our kitchen utensils.

262. Coke Ovens,

At the Duke of Norfolk's colliery near Shef field, several Coke Ovens are built on the side of a hill, occupying spaces formed within the bank. Each oven is circular, 10 feet in diameter in the inside, and the floor is laid with common brick set edgeways. The walls of the oven rise 19 inches above the floor, and the whole is covered by a brick arch which rises three feet five inches more, forming nearly a cone, whose base is 10 feet, and whose apex is two feet, if measured within. This opening of two feet at the top, is left for the convenience of supplying the oven with coal, and to serve as a chimney during the process. The whole height of