"Yes. You see such an enterprise would be absurd. Just notice the man yonder with the long iron rod! He is going to open one of the retorts, take out the old coal--only it is now coke--and put in a fresh supply." A workman in a grimy, leather apron loosened one of the retort doors, and held up a little torch. Immediately a great sheet of flame burst out, and then disappeared. He took the door quite off, and there was a long, narrow oven with an arched top, containing a huge bed of red-hot coals. "What a splendid place to pop corn!" exclaimed Kitty. Papa laughed. "You would find it warm work," said he, "unless you'd a very long handle to your corn-popper." And Kitty thought so too, as she went nearer the fiery furnace. "You see," said Mr. Carter, "these red-hot coals have been changed a great deal by the heat. They have given up all their gas and tar, and are themselves no longer coal, but _coke_. We shovel out this coke and use it as fuel in the furnaces down below to help heat up the next lot. Then new coal is put into the retorts, and they are closed up with iron plates, like that one lying ready on the ground." "It's all muddy 'round the edge," observed Kitty. "Yes, that paste of clay is to make it air-tight. The heat hardens the clay very quickly, so all the little cracks around the edge are plastered up. When the coal is shut up in the ovens, or retorts, the heat, as I just told you, divides it up into the different substances of which it is made; that is, into the coke which you have seen, a black, sticky liquid called tar, the illuminating gas, and more or less ammonia, sulphur, and other things that must be got rid of. Almost all these things are saved and used for one purpose or another, though they may be of no use to us here. If we have more coke than we ourselves need it is sold for fuel. The coal-tar goes for roofing and making sidewalks, or sometimes (though you wouldn't think it possible, as you look at the sticky, bad-smelling, black stuff) in the manufacture of the most lovely dyes, like that which colored Miss Kitty's pink ribbon. The ammonia is used for medicine and all sorts of scientific preparations, in bleaching cloth, and in the printing of calicoes and cambrics." "When the materials of the coal are separated as I told you in the retorts, most of the tar remains behind, and is drawn off; but some gets up the pipes. That large, horizontal cylinder is always nearly half full of it. The gas, which is very light, you know, rises through the upper pipes leading from the retorts, and bubbles up through the tar in the bottom of the cylinder. Then it passes along the farther end of the cylinder, and into the condensing pipes." He opened a door, and they went through into the next room. Here the large pipe which came through the wall of the room they had just left, led to a number of clusters of smaller pipes that were jointed and doubled back and forth upon each other, cob-house fashion.