Roxie Wheeler, age 16, of Dayton, Ohio, for her question:

IS IT POSSIBLE TO MAKE A PERFECT VACUUM?

A vacuum is a space which has no matter in it. The term comes from the Latin word '1vacuus," which means "empty." But strictly speaking, there is no such thing as a perfect vacuum because all space contains some matter.

In practice, we speak of a vacuum as a space from which most of the air or other gas has been taken, and in which the pressure is extremely low. But this is only a partial vacuum.

In the air we are breathing there are about 400 million billion molecules of gas per cubic inch. Man can create partial vacuums in which there are only a few million molecules per inch. But no one has ever created an absolute vacuum    that is, taken all of the air out of an enclosed space.

Actually, we cannot create a perfect vacuum, even with the most powerful air pump or by chemical means. Among the most nearly complete vacuums that have been attained is one in which the pressure of the gas is about one ten thousand millionth of the normal pressure of the air.

Scientists speak of high or low partial vacuums, depending upon how completely the air or any other gas has been taken out of an enclosed space. Any space in which the pressure is about one thousandth of that of the atmosphere is called a vacuum.

The important thing about a vacuum is that it is usually a useful emptiness. For example, if we create a partial vacuum in any space, water or any other fluid may enter into it.

We make use of this principle in drinking soda through a straw. We do not pull the liquid up through the straw. Instead, by sucking on one end of it, we take out some of the air that is in the straw. The air outside exerts pressure on the liquid and forces it up and out through the partial vacuum in the straw.

Pumps which inflate automobile tires make use of this principle also. The vacuum cleaner is another example of the usefulness of a vacuum. In a cleaner, air flows into a partial vacuum and carries dust and dirt into the collection bag of the machine.

In some vacuum tubes in use today, as much air as possible is taken out of the tubes by means of an air pump. Then a chemical called a "getter" is placed inside the tube. This increases the vacuum by combining with a large part of the remaining air inside the tube. The chemical then coats the inside surface of the vacuum tube.

One other important characteristic of a vacuum is the fact that sound cannot cross a high vacuum. This can be seen if we set an alarm clock ringing inside a bell jar in a lab. As the air inside the bell jar is pumped out, the sound of the clock's bell gradually grows less and less until such a high partial vacuum is reached that you can't hear the ringing bell.

Television picture tubes and other kinds of electronic tubes cannot work unless air is pumped from them. The partial vacuum inside the tubes keeps the hot filaments from burning up and allows electrons to flow.