“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.
“When the gas goes through these pipes,” said Mr. Carter, “it gets pretty well cooled down, for the pipes are kept cold by having so great an amount of surface exposed to draughts of air around them. And when the gas is cooled the impurities are cooled too, so that many of them take a liquid form and can be drawn off.”
The next room they entered had a row of great, square chests on each side, as they walked through.
“These are the purifiers,” explained Mr. Carter again. “They are boxes with a great many fan-like shelves inside, projecting out in all directions, and covered thickly with a paste made of lime.”
“Lime like what the masons used when they plastered the new kitchen?” asked Philip.
“About the same thing. The boxes are made air-tight, and the gas enters the first box at one of the lower corners. Then before it can get through the connecting-pipe into the next box, it has to wind its way around among these plates coated with lime. This lime takes up the sulphur and other things that we do not want in the gas, and so by the time it gets through all the boxes it is quite pure and fit to use.”
Then the party all went into the room where the gas was measured. It was a little office with a queer piece of furniture in it; something that looked like a very large drum-shaped clock, with several different dials or faces. This, Mr. Carter said, was the metre or measurer, and by looking at the dials it could be told exactly how much gas was being made every day.