“That is the operation of it,” said his father. “And if you push the wick open a little, so as to separate some of the fibres of it from the rest, then it will take fire more easily still; because the small part which is separated, is more easily heated up to the necessary point, than it was when it was closely in contact with the rest, and so kept cool by it. That is the reason why a thin shaving takes fire so much more easily than the outside of a large piece of wood. The outside of a large piece is kept cool by the parts of the wood behind it, which touch it, while the shaving is heated through very soon.”
“I didn’t know that before,” said Rollo.
“In the cities,” continued his father, “the lamp-lighters, that trim and light the street lamps, always cut the wick off, when they trim the lamps, in a slanting direction, so as to leave a point of the wick projecting up on one side. This point will light very easily, for it stands by itself, somewhat apart from the rest, and so is not kept cool by the rest of the wick. Then, when they put in their great, blazing torch, it heats this point to the degree necessary to inflame the oil very easily.
“There is one thing more I want to tell you, and that will be all I have to say about lamps to-night; and that is, to explain to you the philosophy of putting them out. You must understand that two things are necessary to carry on combustion or burning. First, there must be air; and secondly, the body burning must be kept above a certain degree of heat. Now, if you either suddenly shut off the air from the substance that is burning, or suddenly cool the substance, it will go out. For instance, the wick,—you have to heat it to a certain degree before it will take fire. Now, if, after it is burning, you suddenly cool it below that degree, it will go out; or if you shut out the air from it, then it will go out; for it cannot burn unless it continues hot, and unless it continues to have a supply of air.
“Now, when we blow out a lamp, we stop the burning by cooling it. The cool air which we blow against it, suddenly cools the upper end of the wick below the point of combustion, and so it goes out. On the other hand, when we put it out by an extinguisher, we stop the burning by means of shutting out the air. Either mode will stop the combustion.”
“And how is it when we put on water?” asked Rollo’s mother.
“Why, that is somewhat different from either,” said Mr. Holiday, “or rather it is both combined. There is something very curious in the operation of water upon fire; that I must explain some other day, for now it is time for Rollo to go to bed.”
QUESTIONS.
What was the difficulty in lighting the lamp, as described in the beginning of this chapter? What method did Mr. Holiday say was sometimes adopted in hotels to make the lamps light quick? Why would this make them light more easily? What question did Rollo ask his father in respect to the use of philosophical language? Why did not his father answer the question immediately? Do all substances take fire at the same temperature? What substances take fire with the least degree of heat? What experiment did Mr. Holiday describe which he said would show this very distinctly? Why could not he perform this experiment? Will iron burn? Why do not bars of iron burn in a common fire? How did Mr. Holiday propose to show that iron would burn? How did he propose that Rollo should get some iron filings?