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sinking of a part of the surface of a thing in response to pressure or

a blow, in general to contact. Such bodies are either soft, like

wax, where part of the surface is depressed while the rest remains, or

hard, like copper. Non-impressible bodies are either hard, like

pottery (its surface does not give way and sink in), or liquid, like

water (for though water does give way it is not in a part of it, for

there is a reciprocal change of place of all its parts). Those

impressibles that retain the shape impressed on them and are easily

moulded by the hand are called 'plastic'; those that are not easily

moulded, such as stone or wood, or are easily moulded but do not

retain the shape impressed, like wool or a sponge, are not plastic.

The last group are said to be 'squeezable'. Things are 'squeezable'

when they can contract into themselves under pressure, their surface

sinking in without being broken and without the parts interchanging

position as happens in the case of water. (We speak of pressure when

there is movement and the motor remains in contact with the thing

moved, of impact when the movement is due to the local movement of the

motor.) Those bodies are subject to squeezing which have empty

pores-empty, that is, of the stuff of which the body itself

consists-and that can sink upon the void spaces within them, or rather

upon their pores. For sometimes the pores upon which a body sinks in

are not empty (a wet sponge, for instance, has its pores full). But

the pores, if full, must be full of something softer than the body

itself which is to contract. Examples of things squeezable are the

sponge, wax, flesh. Those things are not squeezable which cannot be

made to contract upon their own pores by pressure, either because they

have no pores or because their pores are full of something too hard.

Thus iron, stone, water and all liquids are incapable of being

squeezed.

Things are tractile when their surface can be made to elongate,

for being drawn out is a movement of the surface, remaining

unbroken, in the direction of the mover. Some things are tractile,

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