In the summer solstice, in the northernmost parts of our island, the sun descends very little below the horizon, and there is no absolute night, for by the twilight a person may distinctly read at midnight. Near the poles of our world, where one day and one night divide the year, there are some weeks of continued twilight, and the sun is several days visible before he actually rises above, and after he has sunk beneath the horizon. See note, page 44 of this volume.

LXV.

Of Dyers' Aqua-Fortis.

This article is used for dissolving tin, to form a mordant for fixing some of the most valuable colours on woollen cloths. In employing this acid, the dyers in the metropolis generally proceed thus: A carboy of aqua-fortis is measured out into a large earthern pan, with from one to two quarts of water for every gallon of the acid, and the whole is well mixed by stirring it with a strong wooden spatula.

Supposing single aqua-fortis be used, the quantity of water employed generally amounts to about one third of the whole; but as no fixed rule can be given, this is left to the discretion of the workman, who apportions the quantity according to the nature and strength of the acid he is using. When the aquafortis and water are thus united, a few handfuls of grained tin are thrown in, in such a manner as nearly to cover the whole of the bottom of the jar. An action immediately commences; and if the aqua-fortis be properly prepared, and the tin uniformly spread, the solution goes on regularly, and no more heat is generated than is necessary to keep up a proper action.

If a froth appears upon the surface, the whole is well stirred with the spatula for a few seconds, as it is known that this frothing is frequently occasioned by the tin lying in heaps at the bottom of the vessel. When this tin is dissolved, more is thrown in, and stirred as before, as occasion requires. If the aqua-fortis be pure, it will now be necessary to add a portion either of common salt or sal-ammoniac to the mixture, which is given in such quantities, and at such intervals, as the appearances of the acid suggest. Here the discretion of the workman is the only guide;-for in this, as in many other cases, practice instructs better than precept.

When rather more than half the usual quantity of tin is dissolved, the liquor begins to turn, as it is termed; that is, a por tion of yellow oxide collects round the mass of tin at the bottom

of the vessel, which when stirred gives a colour to the supernatant liquor. When the aqua-fortis is in a proper state, the colouring always commences at the bottom of the jar, which increases on the further addition of the tin till the whole liquor acquires the colour of rum, or rather of bright amber. If the aqua-fortis be good, these appearances always take place :-if not, the tin may dissolve, but the liquor, as it becomes saturated, will acquire a milky, instead of a yellow, appearance; which shows that the tin is improperly oxidized by the acid, and that such aqua-fortis is totally unfit for use.

It has been doubted whether a solution of tin can be made by means of pure nitric acid; and this much is certain, that the tin will remain only a short time in solution, unless a portion of muriatic acid be also present. The mordant, therefore, that the dyers use for scarlets, is a proper nitro-muriate of tin. In order to produce this, that is, to convert the nitric to the nitro-muriatic acid, it is that common salt or sal-ammoniac is employed; for either of these salts becomes decomposed by the aqua-fortis, and their muriatic acid, thus liberated, performs its part in composing the solution.

Aqua-fortis is employed in a variety of trades, such as brass founders, silver refiners, gilders, leather dyers, and calico printers; but such aqua-fortis would be very improper for dyeing woollen cloths. Nothing can be better known to the dyers in the large way, than that the majority of the aqua-fortis that is manufactured is entirely unfit for dyeing scarlet. Few are there of this occupation, who have not occasionally met with an article of this kind :-and whenever this happens, they know of no remedy but that of laying it by for nine or twelve months, to acquire age, and even then it is often unfit for use.

Formerly, the difficulty of making good dyers' aqua-fortis was so great, that the whole which was consumed in these kingdoms was actually imported from Holland, and there are people now living who remember the time when they never thought of using any but Dutch aqua-fortis. Owing, however, to various causes, the English dyers are now generally supplied from London, or by some particular manufacturers in the country who have paid a peculiar attention to the production of this distinct article.

To those persons who are not sufficiently acquainted with the process of forming the solution of tin, a few directions may perhaps be acceptable.

1st. To prepare the tin, melt it in an iron ladle, and continue it on the fire till it be considerably hotter than its melting point. When in this state, let one man pour it from a height of several feet, into a tub of cold water, while another person constantly agitates the water with a few wooden rods,

IId. In making the mixture of aqua-fortis and water, use as little water as possible, consistent with the nature of the aquafortis employed.

IIId. In adding the first portion of tin, put in such a quantity as by its action shall soon raise the temperature of the liquor fo about 100 degrees of Fahrenheit's thermometer. To effect this, a larger quantity must be added in winter than in summer. If the aqua-fortis be good, it is best to stir the mixture incessantly for about five minutes on the first addition of the tin, which hastens and improves the process. Much, however, of the aqua-fortis that is sold would fire, as it is termed, by this treatment. That is, the violence of the action would decompose the acid, a great part of which would evaporate and be entirely lost in dense red fumes.

IVth. When the mass has acquired the heat before mentioned, the tin should be added in such portions only as will preserve it at about the same temperature.

Vth. Add no salt to the solution till there is an appearance of white oxide collecting at the bottom of the jar. If salt be then added, and stirred with the mixture, this oxide will be taken up by the fluid.

VIth. If the solution of tin be designed for dyeing a perfect flame-coloured scarlet, be careful to add no more salt than is absolutely necessary to keep the tin in solution. If a rose-coloured scarlet be desired, it will be adviseable to add a further portion of salt, according to the colour intended to be produced.

VIIth. When the solution has acquired the proper colour, no more tin should be added; as a saturated solution of tin does not produce such bright colours as one that is in a lower state of saturation. The quantity of tin employed is generally about an eighth of the weight of the aqua-fortis in which it is dissolved.

Description of an Apparatus for collecting Gas from Coal, and of its Use in lighting Manufactories and other large Buildings.

Carburetted hydrogen gas, which is now employed in several parts of the kingdom for lighting large buildings, extensive manufactories, mines, &c., is procured in general from common pit coal by means of simple distillation. A variety of apparatus has been invented for this purpose, some of which are very complicated and difficult to be employed, and others are liable to accidents and explosions. The one which appears to me to have the greatest simplicity, and is at the same time perfectly safe in its application, I have procured a drawing of, and have had engraved for this work:-it may be thus described. The retort, within the furnace, is of cast iron; its larger end has an orifice of several inches' diameter, for the admission of the coal and the withdrawing of the coak; which orifice is made to shut air-tight by means of an iron plug which screws within it; and to the smaller end of the retort a leaden pipe is affixed, in order to lengthen the apparatus and for the purpose of conveying the gas into the reservoir. This leaden pipe is generally bent into the form of an arch, to occasion more surface, and more effectually to cool the gas before it is delivered into the air-vessel or reservoir. The design of cooling the gas in this way is to occasion it to deposit the

tar which always rises with it from the coal, and remains combined with it till it is somewhat reduced in temperature by passing along the leaden tubes. The two straight tubes passing from the extremities of the arch, and furnished with stop cocks, are designed to carry off this tar into a vessel placed beneath to receive it. If the tube connected with the retort be a few feet in length, a part of the tar will run off by the first pipe, and, in passing along the arched part of the tube, the gas will be further cooled, and more tar will be deposited*. The reservoir or receptacle for the gas is constructed on the principle of a common gasometer, and is made with two cylindrical vessels, one inverted within the other. These vessels may be of tin, or copper, or iron, or wood, as the proprietor chooses; and if economy be his first object, the gasometer may even be made with two common casks, the one inverted within the other. The inner vessel should be suspended from the ceiling of the apartment, by a cord or a chain passing over a pulley, and balanced at the other end by a corresponding weight. When the apparatus is thus fixed, water is poured into the outward vessel, so as to fill up the void space between it and the outside of the inner vessel; and the air is then expelled from the latter, by forcing it down to the bottom of the water. The inverted vessel is now become full of water instead of air (the air having passed off at an orifice provided for the pur pose, and furnished with a stop cock), and the whole apparatus is ready for use. After this description, it will be seen by a bare inspection of the drawing, that, whenever the carburetted hydrogen gas is disengaged from the retort, it must rush along the tube, and will rise into the interior of the inverted vessel, which is buoyed up, and continues to rise, so long as the gas continues to accumulate. This suspended and inverted vessel is the only reservoir for the gas, and in this it is safely preserved from escape by means of the water, which is an effectual lute for the apparatus. The quantity of gas in stock is always known by the height of the gasometer, which sinks by the common pressure of the atmosphere, as the gas is used, or withdrawn from beneath it. One of the tubes that rise within the gasometer, is designed to convey the gas into it from the retort, as has been described: the other is intended to convey it to the lamps in the different parts of the apartment. These lamps may be affixed to the main pipe, or smaller pipes may be conducted from thence into other apartments, or from one story of a building to another, as convenience may suggest; for if the gas be lighted it will

The tube which rises nearly perpendicularly from the retort, and goes into the chimney, is designed to carry off the carbonic acid gas and the water which come over in the beginning of the process, and would injure the carburetted hydrogen gas if suffered to mix with it.