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

Carburetted bydrogen 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 coke; 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 purpose, 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. The 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.

ADDITIONAL NOTES.

burn with brilliancy, at any orifice, however distant it may. be from the place where it is prepared. In extensive manufactories, small brass cocks are generally affixed to every aperture, in order to open or shut the communication with the gasometer, at pleasure, that there may be no waste of gas, and that such a number of lights may be had at all times as the manufactories may require.

Whenever a charge is worked off, the retort is opened and the residuum raked out, which is an excellent coke, equal in value to the coal from which it was produced. Besides the coke, a large quantity of tar is soon collected, very useful in some of the Birmingham manufactures, and employed in making a cement for aqueducts, water-cisterns, &c. &c. According to Mr. Northern of Leeds, who has made some accurate experiments on this subject, 50 ounces of coal produce 6 ounces of liquid matter, oil and tar; 26 ounces of coke, and 18 ounces of gas. Where the apparatus is well constructed, and the buildings required to be lighted are large, the process is very economical. Even several hundred pounds are annually saved, in some large establishments, by the use of this gas; particularly in Birmingham, Manchester, and other large manufacturing towns. Some years ago the extensive iron works and the compting-house of Messrs. Bolton and Watt of Soho, near Birmingham, were lighted with it by Mr. William Murdoch of that place, who, as early as the year 1791, was engaged in a series of experiments on carburetted hydrogen gas, and was the first person who employed it in lighting apartments, &c. At the last peace, the whole of the Since then, one buildings at Soho were illuminated by it in a way which occasioned a very grand and unique appearance.

of the streets of London has been lighted by the same means, and the effect produced is such as to delight and satisfy every beholder. It is not only more cleanly than the old method, but the light is more beautiful, and far exceeds in intensity any of the lamps hitherto lighted with oil. See note *, page 257.

I have been induced to note these particulars more in detail than the nature of this work seems to justify, because, as we possess coal in abundance in various parts of the kingdom, any advantages that can be given to our manufactures by its means, appear to me to be of great national importance.

LXVII.

Of the Decomposition of the Alkalies.

In the third edition of the Chemical Catechism, page 531, I gave my readers an abridgement from the Philosophical Transactions for 1807, of a very important paper, read by

Sir H. Davy before the Royal Society, on the agencies of galva nism in the decomposition of a variety of alkaline and earthy salts. The experiments which he detailed in that paper were sufficient to produce an alteration in all our former ideas of chemical affinity, as they proved decisively that the formation of all chemical compounds may depend on the electrical state of the materials of which they are composed. Since these he has had the good fortune to make other important discoveries in this ever-fruitful field of research; and in a paper which he read before the Royal Society on the 19th day of November, 1809, he announced that he had succeeded in the decomposition of the fixed alkalies. Having the permission of the author to make use of this very interesting paper, I have now carefully to abridge it, in order to give a correct and succinct account of the important matter which it contains.

In the first attempts which Sir H. Davy made for the decomposition of the fixed alkalies, he entirely failed, in consequence of his having acted upon their aqueous solutions only. He afterwards used potash in the state of igneous fusion, and acted upon it by an electrical power, which was produced from a galvanic battery of 100 plates of 6 inches square, highly charged. Here some brilliant phenomena were produced. A most intense light and a column of flame were exhibited, which seemed to be owing to the development of combustible matter; and when the order was changed, so that the alkali was brought in contact with the negative side of the battery, aëriform globules, which inflamed in the atmosphere, rose through the potash. Being, however, unable to collect the products of decomposition by this means, he had then recourse to pure potash in its usual state, and depended on electricity alone. for its fusion, as well as its decomposition.

A small piece of pure potash, moistened a little by the breath, was placed upon an insulated disc of platina, connected with the negative side of a battery consisting of 100 plates of 6 inches and 150 of 4 inches square, in a state of intense activity, and a platina wire, communicating with the positive side, was brought in contact with the upper surface of the alkali. Under these circumstances a vivid action soon commenced. The potash began to fuse at both its points of electrization, and small globules having a high metallic lustre, and precisely similar in visible characters to quicksilver, appeared, some of which burnt with explosion and bright flame. These globules, which appear to be metallic, are the basis of potash, that alkali being composed of this peculiar base and oxygen only.

Soda, when acted upon in the same manner, exhibited an analogous result, and these effects equally took place in the atmosphere, and when the alkali was acted upon in the vacuum

of an exhausted receiver; but these globules could not in either case be produced from crystallized alkalies. When a globule of the base of potash was exposed to the atmosphere, it immediately attracted oxygen, and a white crust formed upon it, which proved to be pure potash. When the globules were strongly heated and then suspended in oxygen gas, a rapid combustion with a brilliant white flame was produced, and these metallic globules were converted to an alkali, whose weight greatly exceeded that of the combustible matter consumed.

When Sir H. Davy had thus detected the bases of the fixed alkalies, he had considerable difficulty to preserve and confine them, so as to examine their properties and submit them to experiments. He found, however, at length, that in recently distilled naphtha they may be preserved many days, and that their physical properties may be easily examined in the atmosphere, when they are covered by a thin film of it.

The basis of potash, at 60° Fahrenheit, is only imperfectly fluid; at 70° it becomes more fluid; and at 100° its fluidity is perfect, so that different globules may be easily made to run into one. At 50° it becomes a soft and malleable solid, which has the lustre of polished silver; and at about the freezing point of water it becomes harder and brittle, and when broken in fragments exhibits a crystallized texture, of perfect whiteness and high metallic splendour.

To be converted into vapour, it requires a temperature approaching that of the red heat. It is an excellent conductor of heat, and a perfect conductor of electricity.

Resembling the metals in all these properties, it is, however, remarkably different from any of them in specific gravity; for it will not sink in double distilled naphtha, whose specific gravity is only .770, that of water being considered as 1.000. Sir H. Davy has determined by experiment that its specific gravity is to that of mercury as 10 to 223, which gives a proportion to that of water nearly as 6 to 10; so that it is the lightest fluid body known. When this substance is introduced into oxymuriatic acid gas, it burns spontaneously with a bright red light, and muriate of potash is formed. When thrown upon water, it decomposes it with great violence, and instantaneous explosion is produced with brilliant flame, and a solution of pure potash is the result. When a globule is placed upon ice, not even the solid form of the two substances can prevent their union; for it instantly burns with a bright flame, and a deep hole is made in the ice, which is found to contain a solution of potash. When a globule is dropped upon moistened turmeric paper, it instantly burns, and moves rapidly upon the paper, as if in search of moisture, leaving behind it a deep reddish brown trace. So strong