英文科學讀本 第三冊·Lesson 38 Tar

Lesson 38 Tar

You know, Norah, that if you watch a piece of coal burn, you will see some thick, yellow smoke come from it, as well as the blaze, said Fred.

Yes, said Will, "and it was just the same when teacher was making the gas for us in the tobacco pipe the other day. The first thing we saw was some of this same thick yellow smoke, which came puffing out from the stem."

It seems very wonderful to me, said Fred, "how one lesson leads up to another. We have learned a great many new things today by only thinking over the lesson on gas-making."

Just picture to yourself the great retorts in the gas-works. Not only gas, but dense masses of thick smoke are given off by the coal as it burns in them. The long pipes leading from them pass into great tanks of water.

Everything, therefore, smoke as well as gas, must pass through the water.

The gas is lighter than the water, and it bubbles up to the surface and is carried off, to be collected into the great gasometers. The water condenses the smoke, and it falls to the bottom of the tank as a thick black liquid.

What do you think this is, Norah? It is tar. I had no idea what tar was till today. I am glad I know.

Father has given me some tar in this old cup. Let us look at it. It is a thick, black, shiny, sticky liquid, something like molasses to look at. It has a powerful smell and a bitter taste.

Look what happens if I put a drop of it on my hand. I cannot rub it off. I cannot wash it off. Some grease or turpentine will remove it, because tar is soluble in fat and turpentine, but it is insoluble in water.

If I drop a little in this glass of water you will see that it sinks to the bottom. It will not mix with the water. It is heavier than the water. You know I told you that the people at the gas-works always find the tar at the bottom of the tank.

Now watch while I dip this piece of stick in the tar and set light to it. It sends off dense clouds of smoke and blazes up in an instant. It is very inflammable.

As you know this, I don't think it will surprise you to hear what I am now going to tell you. If tar is heated in a closed retort, some of it rises and passes away in the form of vapor.

The vapor is carried along pipes into a cool chamber called a condenser. As it condenses it forms a new liquid, which is very highly inflammable, and is used for lighting purposes. It is called coal-naphtha.

That which is left behind in the retort is the solid substance which we call pitch.

What a wonderful substance coal is after all, said Norah.

SUMMARY

Smoke, as well as gas, is given off from the burning coal at the gas-works. It is all made to pass through water. The gas is very light, and rises to the surface to be carried away in pipes. The smoke is heavy, and sinks to the bottom as tar. Tar is a thick, black, heavy liquid, something like molasses. It has a powerful smell and a bitter taste. It is insoluble in water. It is very inflammable. Coal-naphtha and pitch are got from tar.