"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. [Illustration: KITTY IN THE GAS-WORKS.] "As soon as the gas gets through the purifiers," said he, "it comes, by an iron pipe, in here, and is made to pass through and give an account of itself before any of it is used. And now I suppose you would like to know how it does report its own amount, wouldn't you?" [Illustration: THE METRE.] Philip and Kitty both were sure they did want to know, so he sketched a little plan of the metre on a piece of paper, and then went on to explain it: