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On The Soul   

the air. The latter condition is satisfied when the air impinged

upon does not retreat before the blow, i.e. is not dissipated by it.

That is why it must be struck with a sudden sharp blow, if it is

to sound-the movement of the whip must outrun the dispersion of the

air, just as one might get in a stroke at a heap or whirl of sand as

it was traveling rapidly past.

An echo occurs, when, a mass of air having been unified, bounded,

and prevented from dissipation by the containing walls of a vessel,

the air originally struck by the impinging body and set in movement by

it rebounds from this mass of air like a ball from a wall. It is

probable that in all generation of sound echo takes place, though it

is frequently only indistinctly heard. What happens here must be

analogous to what happens in the case of light; light is always

reflected-otherwise it would not be diffused and outside what was

directly illuminated by the sun there would be blank darkness; but

this reflected light is not always strong enough, as it is when it

is reflected from water, bronze, and other smooth bodies, to cast a

shadow, which is the distinguishing mark by which we recognize light.

It is rightly said that an empty space plays the chief part in the

production of hearing, for what people mean by 'the vacuum' is the

air, which is what causes hearing, when that air is set in movement as

one continuous mass; but owing to its friability it emits no sound,

being dissipated by impinging upon any surface which is not smooth.

When the surface on which it impinges is quite smooth, what is

produced by the original impact is a united mass, a result due to

the smoothness of the surface with which the air is in contact at

the other end.

What has the power of producing sound is what has the power of

setting in movement a single mass of air which is continuous from

the impinging body up to the organ of hearing. The organ of hearing is

physically united with air, and because it is in air, the air inside

is moved concurrently with the air outside. Hence animals do not

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