英文科学读本 第五册·Lesson 27 Distillation

Lesson 27 Distillation

Our investigations into the heating of liquids have led us to consider a very important subject this morning, said Mr. Wilson.

Think of the water boiling in the flask. What would happen if I held this cold slate over the steam as it issues from the water?

The steam, or water-vapor, would be changed back again into drops of liquid water, sir, said Will. "Just so, and if I boil mercury in a test-tube, that liquid will also pass off in invisible vapor, which I can collect again on a cold surface in little round globules of liquid metal. It is so with all these boiling liquids.

What do we call this process of re-converting the vapor into a liquid?

It is known as condensation, sir, said Will.

What causes the condensation?

The vapor is condensed into liquid because the cold body, with which it comes into contact, robs it of the heat which it contained, said Fred. "It is the addition of heat that first evaporates the water, or changes it into vapor. As soon as that heat is taken away, the vapor changes back again into the liquid form. We say it is condensed. By this term we mean that the molecules of vapor, when they are robbed of their heat, contract and crowd closer and closer together, till they assume the liquid form again."

Very good indeed, my boy, said Mr. Wilson. "Now we will illustrate this in another way. Here is some strong brine; I am going to boil it in this retort. As I don't wish the steam from it to escape, I will fit the neck of the retort into the mouth of this flask. I will stand the flask itself in this basin of ice-cold water, and place the Bunsen burner under the retort. Now let us watch the result.

What is the heat doing to the liquid in the retort?

It is heating and boiling it, sir, said Fred, "and when liquids boil they are converted into invisible vapor."

What becomes of the vapor in this case?

It cannot escape, sir; it passes into the flask.

Quite right. Now let us see what happens to it in the flask. The sides of the flask are very cold; they are surrounded by very cold water. How will this affect the water-vapor inside?

The vapor will be re-converted (or condensed) into liquid water again, sir.

Here, you see, a double process is going on. The heat beneath the retort evaporates the water, and the vapor thus formed, after passing into the cold flask, is changed back again, or condensed, into liquid. We can see the liquid in the bottom of the flask. This double process is called distillation. Let us now remove the flask from the retort, and pour out the water. Taste it, Fred."

Why, sir, said Fred, with surprise, "this is pure water; that in the retort is strong brine."

Yes, Fred; the salt is all left behind in the retort. We call this distilled water. We might have done the same with solutions of sugar, alum, soda, or any other soluble substance. We should, in each case, get only pure distilled water in the flask at the end of the process. The variation in the boiling-points of different liquids is turned to useful account in the art of distilling.

An apparatus, called a still, and formed on the principle of our retort and flask, is used for the purpose. Instead of the retort, a strong copper vessel is used, in which the liquid is boiled. The distilled vapors are passed through a long spiral tube, which is placed in a cistern kept constantly full of cold water. Brandy is obtained from wine by distillation. The wine itself consists of water, alcohol, and certain other matters. Alcohol boils and passes off as vapor at about 172℉., but water will not boil below 212°.

When, therefore, the wine is heated in the boiler to 172°, and not allowed to exceed that temperature, the alcohol in it distils over, and passes, in vapor form, down the spiral tube to be condensed. The water, of course, remains behind, because the temperature is never allowed to reach its boiling-point, 212°. Whisky is distilled from malt liquors in the same way; rum from molasses; benzine from paraffin oil or coal-naphtha."