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. Illustration: (‡ TAR FACTORY.) 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.