"What kind of gas?" inquired Alec.
"Oh! such gas as we burn in stores and houses. I've got my pipe already prepared: if I hadn't, I couldn't show you that experiment very well to-day. I got the pipe ready to show to a boy who was coming to see me last week; but he was sick and didn't come, so I didn't use the pipe."
Johnny took a common clay pipe from his closet, and showed Alec and Belle that the top of the ball of the pipe was closed with plaster of Paris.
"I pounded a little piece of bituminous coal, such as they use at the gas-works," said Johnny, "and nearly filled the bowl of the pipe with it; then I wet a little plaster of Paris, and closed the end of the bowl to make it air-tight,—that is, to keep out the oxygen. There are carbon and hydrogen in the coal, and they will both combine with oxygen very quickly at the right degree of heat: the hydrogen will form a flame, and the carbon will look bright as you see it in a piece of burning wood or coal. But you see the pipe is fixed so that the oxygen can't get at the coal at that end."
"Is the flame of a fire or a lamp caused by the burning of hydrogen?" inquired Belle.
"Yes: the flame is the hydrogen combining with oxygen, and the glowing coal or wick is the carbon uniting with oxygen. The gas from the gas-works is the hydrogen of the coal separated from the carbon. When we heat it with a match to set it to uniting with oxygen, we have nothing but a flame. You know the coal is heated in air-tight retorts; it is heated hot enough to burn, but it can't burn because there is no oxygen for it to unite with; but the heat causes the hydrogen to separate from the carbon, and then it finds its way out through the opening in the retort into the pipes, and when it reaches the air at the end of a pipe, you can heat it a little with a match, and it will begin to unite with the oxygen."
"And the coal that is left in the retort is called coke. I have seen it very often," said Alec: "the reason it looks different from coal, and burns differently, then, is because it has lost its hydrogen?"
"Yes," replied Johnny: "almost all ordinary combustibles are composed of carbon and hydrogen,—wood, coal, oil, etc.; and there are a great many other things that oxygen has a great affinity for, and will combine with at the right temperature: the things that it won't combine with are such as have all the oxygen in them that they will contain, like dirt and stones and ashes."
"And how about the pipe?" asked Alec.
"Why, after Sue gets back with the piece of iron, we will go down and set the ball of the pipe in Katie's fire. When it gets hot, we shall see a smoke coming out of the pipe, which will be composed chiefly of hydrogen gas: we will touch a match to it, and there will be a flame at the end of the pipe until all the hydrogen which was in the coal in the ball of the pipe has united with oxygen. That is one way to make gas on a very small scale."