No. 3.
About Combustion.
BY
W. J. ROLFE, A.M.
Combustion is only another name for burning, and burning in all ordinary cases is oxidation, or union with oxygen, one of the gases that make up our atmosphere. It is a chemical change; that is, one by which we get a new substance entirely unlike any of the substances united. Common salt, for instance, is formed by the chemical union of a yellow, bad-smelling gas and a soft silvery metal. When coal and wood are burned, the chief products of the union with oxygen are carbonic acid and water. The former is a colorless gas, and the latter is in the form of invisible vapor, and both go up the chimney and mix with the outer air. The ashes left behind are only what can not be burned or united with the oxygen. If we collect all the products of the burning, together with the ashes, we find that they weigh more than the coal or wood, the increase being exactly equal to the weight of the oxygen consumed. No kind of matter can be destroyed by any power known to us; it may unite with other matter, and take many new forms, but its weight can be neither increased nor diminished. The amount of matter in the universe is always the same.
Oxygen must be heated before it will unite with coal or wood. The air is at all times in contact with them, but they will not burn unless they are first kindled. The chemical process itself, when once started, generally produces heat enough to raise more oxygen to the proper temperature, and thus the combustion is kept up. The point to which the oxygen must be heated varies much with different substances, as is well shown in kindling a coal fire. The heat produced by rubbing a match on a rough surface suffices to make the oxygen unite with the phosphorus on the end of the match; the burning of this causes heat enough for the union of the oxygen with the sulphur, and the burning of the sulphur enough to set the wood of the match on fire. The shavings, the kindling wood, and the charcoal are in turn ignited, and the burning charcoal develops heat enough to enable the oxygen to combine with the hard coal. Each step in the operation requires more heat than the preceding step. This seems a very simple thing now, but the anthracite beds of Pennsylvania long remained useless because no one had found out how to kindle the fuel, and the discovery was at last made half by accident.
There are some forms of combustion which are very unlike ordinary burning, and yet are essentially the same, being cases of union with oxygen. The only difference is that the process goes on slowly instead of rapidly. We know that vegetable and animal substances decay when exposed to the air; and decay is a slow burning. The oxygen of the air gradually combines with the substances, converting them into carbonic acid and water, and leaving only a small remnant of matter as the ashes of the lingering combustion. The heat produced in this case is found to be precisely the same as in ordinary burning, but it is set free so gradually that it escapes our notice.
We know that green wood decays much sooner than dry wood. Indeed, if wood is kept perfectly dry, it will not decay for ages. In the dry climate of Egypt wooden mummy cases have been preserved for more than three thousand years. On the other hand, dry wood burns much quicker than green wood; it is not easy to set the latter on fire. Why this difference, if decay and burning are similar processes? The decay of the green wood is due to the fact that the presence of moisture causes certain changes in portions of the wood, which enable the oxygen to attack it at a low temperature; and the slow combustion, once started, is self-sustaining. But in ordinary burning the temperature must be raised to a certain point before the oxidation can begin, and this point can not be reached until the moisture is evaporated, which uses up a good deal of heat.
This process of decay is continually going on in our bodies; but during life the matter which is burned up is being constantly renewed from the food we eat. The body is not only decaying, as dead animal matter decays, but it is also wearing out. With every motion a part of the muscles is actually consumed, and must be replaced by fresh material. The heat of the body is likewise due to combustion, and must be kept up by proper fuel, like the fires in our stoves and furnaces. The products of all this burning are carbonic acid and water, which pass out of the body through the lungs.
The rusting of metals is a slow combustion, and scientific men have proved that, like decay, it develops heat. Iron can be easily burned in pure oxygen, with the production of intense light and heat. Zinc and some other metals can be burned in the air if heated very hot, and most metals are rapidly consumed in the flame of the oxyhydrogen blow-pipe. Indeed, every form of matter known to us can be burned, unless it has already been burned. All substances belong to one of these two classes—those that will burn, or unite with oxygen; and those that have been burned, or are products of oxidation. Water belongs to the latter class, and so do nearly all the rocks and solid matter of the earth.
Slow burning sometimes becomes rapid, and then we have what is called spontaneous combustion. When cotton or tow which has become soaked with oil is laid aside in heaps, the oxygen of the air begins to unite with it; but the heat developed causes the oxidation to go on faster and faster, until in some cases the mass bursts into a flame. The same thing sometimes takes place in moist hay, the moisture starting the process, as explained above, and the confined heat increasing until it is sufficient to set the heap on fire.
[By special arrangement with the author, the cards contributed to this useful series, by W. J. Rolfe, A.M., formerly Head-Master of the Cambridge High School, will, for the present, first appear in Harper's Young People.]
GETTING WEIGHED.
DAVE'S GREAT LUNCH.
BY J. B. MARSHALL.
It was the great day at the State Fair, and the sidewalks were nearly deserted as Dave Burt went down Main Street toward the post-office. As Dave approached the Town Hall, or the City Hall, as the good people of Rawley were pleased to call that fine building, he glanced up at it, and saw Mr. William Henry Barrington, the great lawyer, standing at one of the large windows of his office. Mr. Barrington was frowning, and looked up the street and down it as if impatiently waiting for some one.
"I'll bet he's mad 'cause he can't go to the fair," thought Dave.
A few days before, Billy Barrington, a nephew, had been telling the boys of that fine office, with its brass-studded revolving chairs, great bookcases of books, and a private room where the great lawyer ate his dinner, which was sent up to him on a dumb-waiter from the restaurant in the basement of the City Hall the moment he touched an electric bell.
Dave was recalling all the delightful possibilities of such a room, when click! went something on the pavement before him.
"A penknife," said he, picking up the article, and then, looking in vain among the branches of the tree for its owner. Examining the knife, he noticed a slip of paper shut in under the largest blade, and on which was written:
"Five Dollars Reward! I am on the City Hall roof, and can't get down, as the spring-latch door has blown closed. Please send the janitor to release me.
"Charles M. Wilson."
"Why, he's our Governor!" said astonished Dave, aloud, and started to look for the janitor. Dave had been on the roof with his father only the day previous, and knew just how the door would act if it was not fastened back.
Stout old Billy Simms, the janitor, in his shirt sleeves, had comfortably propped himself back in an arm-chair to take a nap, when rap-rap-rap sounded on the door. Billy's "office," as he called it, was on the ground-floor of the City Hall.
"Well, boy, what's wanted?" gruffly demanded old Billy, having opened the door and discovered Dave.
"Why, the Governor's shut out on the roof, and can't get down," said Dave, handing Billy the paper. "He must have been looking at the Fair Grounds."
Old Billy lowered his great silver-rimmed glasses from his forehead to his nose, and read the paper. He gazed for a moment in a queer way over his glasses at Dave, and then laying his hand pretty heavily on Dave's shoulder, said, "Come with me."
"I haven't time; and, besides, I don't want any reward," answered Dave.
There was a small room, or closet, back of Billy's "office," toward which he moved, holding fast to Dave.
Remembering that the old janitor was rather deaf, Dave then formed his hands in the shape of a trumpet and shouted in the direction of Billy's right ear, "I say, Billy, I haven't time to go with you."
"Don't you call me Billy, you young rascal!" fiercely exclaimed the old man. "My name's Mr. William Simms."
Before Dave could make reply he felt himself shaken, pushed into the closet, and saw the door nearly closed.
"There, you've played that trick once too often," said old Billy. "It's downright murder in you boys to try and fool me into going up seven long flights of steps on an awful hot day like this."
"I did find that paper," said Dave, indignantly.
"Don't tell me you're innocent; you're a desperate character," said old Billy, slamming to the door, and turning the key. "Now," continued he, shouting through the key-hole, "I'll leave you in there two or three hours to think what a dreadful thing it is to try and trick an old rheumatic veteran."
The closet, Dave saw, was where Billy kept his brooms and brushes; the ceiling was very high, and a small round window far up on the wall furnished the light. At the back of the closet was a small sliding shutter, which, after considerable trouble, Dave managed to push up, hoping he might escape through it into another room. It disclosed a dark, square funnel, that seemed to extend far down below and far up above him, and suspended in which were several wire ropes.
"It must be the funnel where the dumb-waiter slides," thought Dave, and he caught hold of the nearest rope, pulling and shaking it to attract attention, and calling loudly at the same time. At once he heard a tinkle-tinkle of a small bell up the dark funnel; and then a scraping sound from the same direction, seeming to draw nearer him. Directly the dumb-waiter cage was seen descending, and Dave held fast to the wire rope until the cage was within a short distance of his hand.
When the cage ceased to move he climbed into it by aid of a chair, and curled himself up, hoping to go down into the restaurant. There was a wire running through the cage, and supposing it to be the same he had been previously holding, he pulled at it with both hands.
The cage began to move; but in place of going down, it began to move upward. Dave was frightened; but before he could decide what he ought to do, the cage had passed above the open shutter, and went on scraping between four dark wooden walls. Up and up went the cage, until Dave felt that he had traversed a distance far more than enough to have carried him to the very tip of the lightning-rod on the City Hall cupola.
Suddenly he saw a thin streak of light before him, and quickly releasing the wire, the cage moved a little further, and then came to a stop. Dave lost no time in waiting to drum on the door, partition, or whatever it was before him, and loudly called:
"Hello! Let me out! let me out!"
In a moment there was the sound of quick feet, a sliding shutter was pushed aside, and such a flood of light shone into Dave's face that before he could get the dazzle out of his eyes some one carefully lifted him out of the cage, and stood him on his feet.
"What ever possessed you to take a ride in that carriage?" asked a pleasant voice.
Dave shaded his eyes, and saw that he was standing before Mr. Barrington in his private office.
"It's all that old Billy Simms's fault," said Dave, hotly, "and he ought to be arrested. I found a paper on the pavement that said a man was locked out on the City Hall roof, and please somebody come and open the door for him. But when I gave it to Billy, he just locked me up in a room, and said I was playing a trick on him, and the Governor wasn't on the roof. Then I opened a shutter, and—"
"The Governor fastened out on the roof!" said Mr. Barrington. "I've been waiting an hour for him to come and eat lunch with me, but this accounts for his absence. Sit down, my little man." Then Mr. Barrington stepped into another room, where Dave heard him send one of his law clerks to release the Governor.
"I see you are Captain Burt's son David," said Mr. Barrington, returning. "Simms has treated you very badly; but come—you must be hungry, being shut up in that dark hole—sit down here at the table, and eat some lunch. There will be plenty for the Governor."
Dave excused himself, having already dined.
"Then I know what you will eat—a Neapolitan ice."
The door opened, and the Governor entered, looking as though he was nearly roasted; and in a moment Mr. Barrington had explained to him how Dave had tried to have him released.
"I'm many times obliged to you, David," said the Governor, shaking Dave's hand, and making him feel very proud.
The Governor was too near broiled himself to feel like eating lunch, but the ices appearing, he helped Mr. Barrington and Dave to eat them.
When the ices were eaten, the Governor wished to give Dave the five dollars, as promised, but he was very, very sure he ought not to take it. In a few days, however, there came to Captain Burt's house a package of books, marked "Master David Burt," and within was a note with the compliments of the Governor.