on 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.

"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, an