The gasometer is made of plates of iron rivetted together. The lower part of this cylinder (see [figs. 4] and [5]) is received into a circular deep channel cut in the ground (b b, [fig. 5]) and filled with water, so that the gasometer d, which is suspended and balanced by means of the chains and weights e e, may be raised or depressed at pleasure. When the gasometer is quite down, the inside of its top, a, rests on the ground; when the gas enters, it raises it up till the lower edge is but a few inches below the water, and is prevented from rising higher by frames which support it; when some of the weights, d d, are removed, the huge cylinder, a, slowly sinks down and forces the gas through the tubes, c c, to be burnt by thousands of consumers. About four cubic feet of gas is produced from every pound of coal used; the gas is lighter than air in the proportion of 650 to 1000, and it is owing to this lightness that balloons ascend when filled with it, not from any peculiar power it has of ascending, but because the air, being heavier, presses downwards and forces it up. When gas is mixed with air it becomes very explosive, and when any escape is suspected in a room, the windows of the room should be opened at the top, and the door also opened, before any light is introduced; a few minutes will then suffice to ensure safety.
One ton of good coal produces—
| 1 | Chaldron of Coke, | weighing | 1494 | lbs. |
| 12 | Gallons of Tar | ” | 135 | ” |
| 12 | Gallons Ammoniacal liquor | 100 | ” | |
| 9500 | Cubic feet of Gas, | weighing | 291 | ” |
| Loss (chiefly water) | ” | 220 | ” | |
| 2240 | lbs. | |||
| A cubic foot of gas weighs 514 and a-half grains. | ||||
TAR.
Tar is one of the results left in the distillation both of wood and coal; in places where wood is plentiful and tar in request, it is produced by burning the wood for that purpose; and in some of the pits in which charcoal is produced, an arrangement is made to collect the tar also. Coal-tar and wood-tar are different in some respects, and are both distilled to procure the napthas which bear the respective names. From wood-tar, creasöte is also extracted, and it is this substance which gives the peculiar tarry flavor to provisions cured or preserved by being smoked over wood fires, such as ham, bacon, or herrings. Tar is used as a sort of paint for covering wood-work and cordage, when much exposed to wet, which it resists better than anything else at the same price; but the tar chiefly used for these purposes, is that produced by burning fir or deal wood and condensing the tar in a pit below the stack of wood, it is called Stockholm tar, as it comes chiefly from that place.
ACETIC ACID, OR WOOD VINEGAR.
Acetic acid forms a considerable article of commerce. It is not only used in medicine and the culinary art, but is extensively employed in forming acetate of iron for dyeing and calico printing. To prepare it, large iron cylinders, about eight feet long and three in diameter, are embedded in brickwork in a row, and in such a manner that furnaces placed below may heat them red-hot; these cylinders have a tube leading from each into a main tube, where the liquid products from each cylinder are received for condensation; the other end of the cylinder has a plate of iron fitting closely to it. The cylinders are filled with logs of wood, either oak, beech, birch, or ash, the door is closely fastened and the joints smeared with clay; the fires are now lighted and kept up all day, till the cylinders are red-hot; at night they are allowed to cool. About seven or eight hundred weight of wood is put into each cylinder. In the morning, the charcoal, into which the wood is now converted, is withdrawn, and a fresh charge supplied; from this charge of wood about thirty or forty gallons of liquid is condensed in the main tube from each cylinder, the remainder being charcoal and gases which pass off; the liquid is acid, brown, and very offensive, and contains acetic acid, tar, and several other ingredients, among which may be named creasote; it is from this source all the creasote, so famous for the cure of toothache, is obtained. The next process is to purify this liquid; it is first distilled, and this separates much tar, it is then mixed with lime, evaporated to dryness, and heated to expel the remaining tar and other impurities; it is next mixed with sulphate of soda and water and the whole stirred together, the soda, now in unison with the acetic acid, is washed out from the lime and strained quite clear; it is afterwards evaporated till it crystallizes, and vitriol (sulphuric acid) then added; finally, the acetic acid is distilled over, and the acid left in unison with the soda, forming sulphate of soda, to be used in a similar process for the next batch of acid; the acetic acid is now quite colorless, transparent, and very sour, possessing a fragrant and agreeable smell. This acid is not pure acetic acid, but contains a considerable quantity of water. The acetic acid of commerce, mixed with seven times its bulk of water, forms an acid of about the strength of malt vinegar, perfectly wholesome, and, to many, more agreeable as a condiment.