# Man's physical universe

THE KINETIC- MOLECULAR THEORY 235

2. What is the size of a molecule?

Answer: Molecules are extremely small — so small, indeed, that they

cannot be seen by the most powerful microscope.

A row of 40,000,000 water molecules would measure an inch. If a

liter of water were poured into the ocean and each water molecule were

labeled in such a way that it could be identified a few million years

later, after all the water of the earth was thoroughly and uniformly

mixed, any liter of water then taken would be found to contain 2220 of

the original molecules.

It has been calculated that if the molecules of one cubic inch of air

were each changed to a grain of sand, the resulting sand would fill a

trench a mile wide and three feet deep reaching from New York to

San Francisco.

The British physicist, Aston, gives the following illustration:

"If you made a hole in an ordinary evacuated electric light bulb

which would allow the molecules of air to pass in at a rate of one million

per second, it would be about a hundred million years before the bulb

would be filled."

3. How far apart are the molecules?

Answer: The molecules of gases are very far apart in relation to their

size, inasmuch as the actual space occupied by the molecules of a given

volume of gas is only a very small portion of this volume.

4. Are all molecules the same size?

Answer: No. The molecules of a given substance have the same

average size, but molecules of different substances have different

average sizes. Hydrogen molecules, as compared with certain very

large protein molecules, would be in the ratio of 1 to 33,000.

5. Do molecules move?

Answer: Yes, molecules of gases at ordinary temperatures move

very rapidly. The average speed of hydrogen molecules at room temperature

is about a mile per second.

6. Do all molecules of a given substance under the same conditions

move with the same speed?

Answer: No. Most of the molecules move with speeds very near

the average speed, but a few move with speeds very much more or very

much less than the average speed.

Today, according to the theory of probability, the many well-known

laws, such as the gas laws, are expressions of probable tendencies,

inasmuch as they deal with statistical averages rather than with

individual particles. Thus it can be predicted within narrow limits

how many babies will die in the United States in a year. But we cannot

foretell how long one particular baby will live.

The theory of probability, formulated by Pascal and Fermat in the

seventeenth century, has been used to estimate the errors in physical

measurement and to rationalize insurance and statistics.

The Belgian astronomer Quetelet (1796-1874) showed that the chest

measurements of Scottish soldiers gave a curve of variation that is

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