英文科學(xué)讀本 第六冊·Lesson 32 Carbon

Lesson 32 Carbon

You all know this light, brittle, porous, black substance, began Mr. Wilson. "It is charcoal, or, to use the language of the chemist, carbon. Carbon, then, is to be the subject of our lesson this morning.

Carbon is the main basis of all vegetable matter. If a plant be carefully dried, so as to drive off all the water it contains, more than half of what remains will be this substance, carbon.

Mr. Wilson then proceeded to show the class some carbon, which he had prepared in readiness for the lesson.

Look at this piece of iron gas-pipe, he said, "it is full of carbon. This morning I filled it with chips of wood, closed up both ends with clay, bored a few holes in the clay stopping, and then put the pipe into the hottest and brightest part of the fire. After a time I removed it from the fire, and left it to cool.

Let us now break away the clay, and see what we have inside. The wood has been changed in the pipe into charcoal or carbon, exactly like the piece I showed you just now. You must clearly understand that the wood has not been burned. If we put some similar chips on the fire, and let them burn in contact with the oxygen of the air, we shall notice that the whole of the substance will disappear, leaving nothing but a little white ash. By placing the wood in the closed pipe, within the fire, we caused the wood to decompose or separate into its constituents. Some of these constituents passed off, in the form of gases, through the holes in the clay, but the carbon itself, being closed in, was not allowed to come into contact with the oxygen of the air, and was therefore not consumed.

We have already said that carbon is the chief constituent of all plants. We obtained it from the wood chips, and we could obtain it from any vegetable matter. Now, think for a moment of coal, which, you know, is formed from the buried remains of the forests of far-distant ages. The vegetation of those times, like the vegetation we meet with now, had carbon as its chief constituent; hence the coal which we burn today consists very largely of carbon. If we heat coal in a closed vessel, as we did the chips of wood, we shall find this constituent, carbon, left behind in the form of coke, while all the other constituents pass off as gases."

Mr. Wilson next crushed a few pieces of lump-sugar in a mortar, put the powder into a saucer, made a syrup by pouring hot water on it, and then added a few drops of sulphuric acid. The syrup instantly turned color, and swelled up into a black mass, resembling a piece of coke.

Sugar, you know, said he, when the class had got over their surprise, "is a vegetable substance; it is prepared from the sweet juice of the sugar-cane, beetroot, maple, and other plants. It is clear that the juice contains carbon, for the black mass you now see is simply carbon which has been set free by the sulphuric acid.

Turning next to the animal world, we know that man and all animals feed directly or indirectly on plants. Hence the carbon of the plant is used to build up the tissues of the animal. There is carbon in all the tissues of the body; a special kind of charcoal, known as ivory black, is prepared from bones. Fat consists very largely of carbon, and the presence of carbon can soon be detected in the charring of the joint of meat before the fire, if the cook forgets to turn it.

Among mineral substances carbon forms an important constituent in limestone and other rocks, and in plumbago, graphite, or black-lead, as it is sometimes called. The beautiful diamond—the most precious and brilliant of all gems—is simply a special form of carbon. Let us now have an old experiment, performed in a somewhat different way. I have here a jar of oxygen which I prepared in readiness for our lesson. I will place this piece of carbon in the deflagrating spoon, heat it to redness in the flame of the Bunsen burner, and then plunge it into the oxygen.

The combustion goes on with greatly increased brilliancy, showing the powerful affinity that exists between carbon and oxygen. This is why, in preparing the carbon for the lesson, I put the chips of wood into the closed pipe. If they had been in contact with the oxygen of the air, the whole of the substance of the wood would have been consumed. You have already become familiar with the product formed by this combustion of carbon in oxygen. You call it carbonic acid gas. The chemist usually calls it carbon dioxide. You are aware that all substances which combine in this way with oxygen form oxides. This is called dioxide because it contains two parts of oxygen to one of carbon. Di means two. Our jar, then, is now filled with carbon dioxide. You know many of the properties of this gas already. Let us learn a little more about it by means of an experiment. I have here a solution of blue litmus, which is a vegetable coloring matter. We will pour some of the solution into the jar of carbon dioxide, and note what takes place. The blue color instantly changes to a bright wine-red when I shake the jar. Just one more experiment before we close the lesson. I will close the mouth of the jar with my hand, and place it, mouth downwards, in this bowl of water. As soon as I take my hand away, some of the water rushes up into the jar.

Carbon dioxide is very soluble in water. Water will dissolve its own bulk of this gas. This is why the water rushed up into the bottle.