Rollo did not think to ask his father the reason why the philosophers use such learned language, or, as he expressed it, such hard words, for several days. Perhaps he never would have thought of it again, if his father had not happened to use the word combustible one evening, which reminded him of the term combustion, which Jonas had used. The occasion on which his father used the word was this,—

One evening, Rollo’s mother was trying to light a little lamp, to go into her bedroom for something that she wanted. There were, usually, in a little vase upon the mantel-piece, some lamp-lighters, which were long, slender rolls of paper, that Rollo had rolled up for this purpose. They were kept in this vase upon the mantel-piece in order to be always ready for use. But the vase was now empty. The last lamp-lighter had been used; and so Rollo’s mother folded up a small piece of paper, and attempted to light the little lamp, which she was going to carry into the bedroom, with that.

But the wick would not take fire, and Rollo saw that, while his mother was continuing her efforts to make it burn, the flame of the paper was gradually creeping up nearer and nearer to her fingers. At last, finding that there would soon be danger of burning her fingers, she walked across the room towards a window which was open, still endeavoring to light the lamp. But it was all in vain. She reached the window just in time to throw the end of the paper out, and save her fingers from being burned.

“Why won’t it light?” said Rollo.

Rollo’s father was sitting upon the sofa, taking his rest after the labors of the day; and when he saw that the lamp failed of being lighted, he said,—

“You will have to get a longer lamp-lighter, unless you have got some spirits of turpentine to put upon the wick.”

“Spirits of turpentine?” repeated Rollo.

“Yes,” said his father. “In hotels, where they have a great many lamps to light, they have a little bottle of spirits of turpentine with a wire running down into it; and, when they take out the wire, a little drop of the spirits of turpentine hangs to the end of it, and they touch this to the wick, and then it will light very quick.”

“Why, sir?” asked Rollo.

“Because spirits of turpentine is very combustible, or rather inflammable.”

“That means it will burn very easily, I suppose,” said Rollo.

“Yes,” replied his father.

“That makes me think of something Jonas said, which I was going to ask you,” said Rollo. “He said that, in books, burning was always called combustion, and I told him I meant to ask you why they couldn’t as well call it burning.”

“I don’t think that Jonas said exactly that,” said his father.

“Yes, sir, he did,” replied Rollo; “at least, I understood him so.”

“It is true, no doubt,” added his father, “that, in philosophical books, philosophical terms are very often used, instead of the common language which we ordinarily employ.”

“Why are they, father?” said Rollo. “I think the common words are a great deal easier to understand.”

“Yes,” said his father, “but they are not precise in their signification. They are vague and ambiguous, and so philosophers, when they wish to speak accurately, employ other terms, which have an exact signification.”

Rollo looked perplexed. He did not understand at all what his father meant. In the meantime, his mother had brought a fresh bundle of lamp-lighters from the closet, and had lighted her lamp with one of them, and was just going away. As she was going out, however, she said to her husband,—

“Please to wait a minute, until I come back, for I should like to hear what you are going to say.”

“Well,” said he; “and you, Rollo, may come and sit down by me, and I will explain it to you when mother comes back.”

So Rollo came and took a seat on the sofa by the side of his father, saying,—

“Father, I wish you would have a bottle of spirits of turpentine for us to light our lamps by.”

“It is not of much advantage in a family,” said his father, “where the lamps are lighted in various parts of the house, and only a few in all to be lighted. But where there are a great many, it is quite a saving of time to have a little spirits of turpentine to tip the wicks with. In an illumination they always touch the wicks so, and by that means they can light up suddenly.”

“But, father, why will the wick light any quicker?”

“Why, different substances take fire at different temperatures. For instance, if you were to put a little heap of sulphur, and another little heap of sawdust, on a shovel together, and put them over a fire, so as to heat them both equally, the sulphur would take fire very soon, but the sawdust would not until the shovel was very nearly red hot. So if you were to put oil in a little kettle over the fire, and spirits of turpentine in another kettle, and have the fire the same under both, the spirits of turpentine would inflame long before the oil. There is a great difference in different substances, in regard to the temperature at which they inflame.”

“What do you mean by temperature, father?” said Rollo.

“Why, heat,” said his father.

“Then why don’t you say heat?” said Rollo.

His father laughed.

“What are you laughing at, father?” said Rollo.

“Why, that’s the same question that you asked me at first, and I promised to wait till mother came before I explained it. So we will wait until she comes.”

They did not have to wait long, for Rollo’s mother soon returned; and she put out her lamp by means of a little extinguisher which was attached to the stem of the lamp itself. Then she sat down at the table, by the light of a great lamp which was burning upon it, and took out her work.

Rollo’s father then repeated to her what he had just been telling Rollo, namely, that different substances took fire at different degrees of heat; and he said that it would be a very interesting experiment to take a long iron bar, and put a small quantity of several different substances upon it, in a row, and then heat the bar gradually, from end to end all alike, until it was very hot, and so see in what order the various substances would take fire.

“I would have,” said he, “phosphorus, sulphur, sawdust, charcoal, saltpetre, oil,—we should have to make a little hollow in the iron for the oil,—alcohol, spirits of turpentine, and perhaps other things. The phosphorus would take fire first, I suppose, and then perhaps the sulphur, and others in succession.”

“Well, father,” said Rollo, “I wish you would. I should like to see the experiment very much.”

“No,” said his father, “I cannot actually try such an experiment as that. I could not get such a bar very conveniently; and, if I had the bar and all the substances, I could not heat the bar exactly equally. It could not be done very well, except in a chemical laboratory. But it would be a very pretty experiment, if it could be performed.”

“Is there a very great difference,” said Rollo’s mother, “in the degree of heat necessary to set fire to these different things?”

“Yes,” said Mr. Holiday, “I believe the difference is very great. Phosphorus inflames below the heat of boiling water, but it takes almost a red heat to set wood on fire. And iron will not take fire till it is white hot.”

“Iron?” said Rollo, with surprise.

“Yes,” said his father, “iron will take fire and burn as well as wood, if you heat it hot enough.”

“I never knew that,” said Rollo.

“The ends of the tongs and of the andirons do not burn,” said his father, “simply because the fire is never hot enough to set such a large piece of iron on fire. But if we heat the end of a bar of iron very hot indeed in a furnace, it will take fire and burn; and so, if we take a very minute piece of iron, as big as the point of a pin, a common fire would be sufficient to heat that hot enough to set it on fire.”

“Well, father,” said Rollo, “let us try it.”

“If we had some iron filings, we might sprinkle them in the fire, or even in the flame of a lamp, and they would burn.”

“I wish I had some filings,” said Rollo.

“Yes,” said his father, “they burn beautifully.”

“How can I get some?” asked Rollo.

“You can get some at a blacksmith’s shop,” said his father. “The filings commonly accumulate behind the vice, and you can get plenty of them there. The next time you go by a blacksmith’s shop, you had better go in and ask him to give you some.”

“Well,” said Rollo, “so I will.”

“And now do you understand,” said his father, “why it is that you can light a lamp more easily when there is a little spirits of turpentine on the wick?”

“Yes, sir,” said Rollo. “The spirits of turpentine need not get so hot before it catches fire, and so you don’t have to hold the lamp-lighter so long, and burn your fingers.”

“Will oil always take fire when it gets to a certain degree of heat?” asked Rollo’s mother.

“Yes,” said his father, “I suppose so.”

“And yet,” said she, “the lamp seems to take fire much more easily at some times, than at others.”

“Yes,” said Mr. Holiday, “that is true. If the wick is cut square across, and rises up only a very little way above the tube, it is very difficult to light it, because the tube itself and the oil below keep the upper end of the wick cool. It is very hard to heat it, in that case, hot enough to set it on fire. But if the wick projects considerably out of the tube, then it is out of the way of the cooling influence of the metal, and you can heat the upper end more easily.”

“I never thought of that,” said Rollo.

“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?