much
as they did from Germany. The Great Salt Lake of Utah is a relic of a
great body of water. If it be true that waters once covered our
Western States, there may be buried deposits of potash there, and
to-day the search for the hidden treasure is going on with the energy
and enthusiasm characteristic of America.

Another probable source of potash is seaweed. The sea is a vast
reservoir of potash, and seaweed, especially the giant kelp, absorbs
large quantities of this potash. A ton of dried kelp (dried by sun and
wind) contains about 500 pounds of pure potash. The kelps are
abundant, covering thousands of square miles in the Pacific Ocean,
from Mexico to the Arctic Ocean.

CHAPTER XXVII

SOUND

249. The Senses. All the information which we possess of the world
around us comes to us through the use of the senses of sight, hearing,
taste, touch, and smell. Of the five senses, sight and hearing are
generally considered the most valuable. In preceding Chapters we
studied the important facts relative to light and the power of vision;
it remains for us to study Sound as we studied Light, and to learn
what we can of sound and the power to hear.

250. How Sound is Produced. If one investigates the source of any
sound, he will always find that it is due to motion of some kind. A
sudden noise is traced to the fall of an object, or to an explosion,
or to a collision; in fact, is due to the motion of matter. A piano
gives out sound whenever a player strikes the keys and sets in motion
the various wires within the piano; speech and song are caused by the
motion of chest, vocal cords, and lips.

[Illustration: FIG. 164.--Sprays of water show that the fork is in
motion.]

If a large dinner bell is rung, its motion or vibration may be felt on
touching it with the finger. If a tuning fork is made to give forth
sound by striking it against the knee, or hitting it with a rubber
hammer, and is then touched to the surface of water, small sprays of
water will be thrown out, showing that the prongs of the fork are in
rapid motion. (A rubber hammer is made by putting a piece of glass
tubing through a rubber cork.)

If a light cork ball on the end of a thread is brought in contact with
a sounding fork, the ball does not remain at rest, but vibrates back
and forth, being driven by the moving prongs.

[Illustration: FIG. 165.--The ball does not remain at rest]

These simple facts lead us to conclude that all sound is due