英文科學(xué)讀本 第六冊(cè)·Lesson 15 Combustion

Lesson 15 Combustion

Mr. Wilson commenced the next lesson by repeating some of the old experiments with oxygen. He had several jars of the gas prepared beforehand, and he plunged into them, one by one, in succession, the burning splinter of wood and the pieces of carbon, sulphur, and phosphorus, just as he had done in the earlier lessons. In each case, too, he allowed the substance to burn first in the air of the room, to show the striking difference in effect when he plunged them into the pure oxygen.

Now why do these things burn at all in the air? he asked.

Because the air contains oxygen, said several boys at once.

Think over our last lesson on chemical combination, and tell me what the burning means in each case.

It means, sir, replied Fred, "that each of these substances has an affinity or attraction for oxygen and, because of that affinity, abstracts oxygen from the air, and enters into chemical combination with it to form a new compound substance."

Excellent, my lad, said Mr. Wilson. "Now, of course you noticed, too, that the wood and the carbon did not burn as briskly as the sulphur, nor the sulphur as briskly as the phosphorus. The reason is that the chemical attraction of all these substances for oxygen is not equally strong. In phosphorus, as we have seen, it is so strong that, unless the element is kept in water, it takes fire and burns away. You must remember that the burning which took place in the air and in the pure oxygen is precisely the same in kind; it differs only in degree. In either case it was entirely due to the presence of oxygen. Each of these substances combined with oxygen to form a new compound, and while combining gave off more or less heat and light. This is just what we mean by combustion.

Suppose we have another experiment now; I am going to show you some iron in the very act of combining with oxygen to form a new compound.

Mr. Wilson then took a piece of very thin iron wire, and, after coiling it round a ruler to make it into a sort of spiral, he fixed one end of it into a cork, and dipped the other end, red-hot, into some sulphur. He then showed the boys a large, bottomless, glass bell-jar which was standing on an iron tray on the table.

This jar, said he, "is filled with oxygen. I shall remove the stopper presently, and fix in its place this cork, which holds our coil of wire. The cork fits the neck of the jar exactly. Watch what happens." When all was ready, he first held the sulphur-tipped end of the wire in the flame for a moment, and then, while the sulphur was blazing, he rapidly took out the stopper and put the cork in its place as he had said. The instant the iron wire was plunged into the oxygen in this way, it burst into a most dazzlingly brilliant flame; intense heat was given out, and white-hot, fused drops of some newly-formed substance were seen to fall continually as the burning proceeded.

Now, said Mr. Wilson, "we see the same thing taking place with this metal—iron—as we have seen with other substances. Chemical action is going on fiercely in the oxygen. The iron and the oxygen are entering into chemical combination, and those fused drops are the new compound which is being formed. In other words, combustion is going on between the iron and the oxygen.

As this kind of chemical action cannot take place without the presence of oxygen, we always speak of this gas as the great agent of combustion. I think we now clearly understand that, whenever combustion is going on, a new compound is being formed, as the product of that combustion.

Take for example the piece of carbon, which burns, as you know, either in the air or in pure oxygen. Why does the carbon burn? Carbon, when once it is heated, has a strong affinity for oxygen. It combines with oxygen to form a new substance—carbonic acid gas—and while the combination is going forward, heat and light are produced. We have already learned that coal, wood, coke, peat—everything we use as fuel—as well as the coal-gas, paraffin and other oil, and candles, which we use for lighting purposes, consist largely of the element carbon. None of them would burn in an atmosphere deprived of its oxygen.

The burning of any one of these illustrates clearly what is meant by the term combustion. It is simply the chemical combination of the substance which is being burned with the oxygen in the air around it. It is important to keep clearly in mind the fact that the carbon so burned is not destroyed. It is merely changed into a new form in combination with oxygen as carbonic acid gas. We utilize for our own purpose the heat and light given out by these substances during combustion; while the poisonous product, carbonic acid, is made to disperse as rapidly as possible.

Whenever combustion takes place, and the substance unites chemically with oxygen, we use one word to describe the new compound that is formed. We call it an oxide. For example, the piece of sulphur burned with a pale blue flame, and gave off the well-known powerful smell with which we are familiar when we strike a lucifer match. This smell came from the newly-formed compound of the sulphur and oxygen, which we may call an oxide of sulphur, So with the burning phosphorus and the iron wire. In one case the product of the combustion was an oxide of phosphorus; in the other an oxide of iron. In like manner we may have oxides of lead, zinc, copper, tin, mercury, calcium, etc.

You have seen the red oxide of mercury, and you know, too, that quicklime is the oxide of calcium. This rusty piece of iron is covered with the oxide of that metal, for the rust is the oxide of iron. If I scrape off some of this reddish-brown powder from the surface, I know that I have a compound formed of iron and oxygen. It was formed as other oxides are formed; the only difference is that the combustion was so slow, owing to the small quantity of oxygen in the air, that the usual heat and light were not evolved during the process.