2. Solidity fills space .
This is the idea which belongs to body,
whereby we conceive it to fill space. The idea of which filling of
space is, that where we imagine any space taken up by a solid
substance, we conceive it so to possess it, that it excludes all other
solid substances; and will for ever hinder any other two bodies,
that move towards one another in a straight line, from coming to touch
one another, unless it removes from between them in a line not
parallel to that which they move in. This idea of it, the bodies which
we ordinarily handle sufficiently furnish us with.
3. Distinct from space .
This resistance, whereby it keeps other
bodies out of the space which it possesses, is so great, that no
force, how great soever, can surmount it. All the bodies in the world,
pressing a drop of water on all sides, will never be able to
overcome the resistance which it will make, soft as it is, to their
approaching one another, till it be removed out of their way:
whereby our idea of solidity is distinguished both from pure space,
which is capable neither of resistance nor motion; and from the
ordinary idea of hardness. For a man may conceive two bodies at a
distance, so as they may approach one another, without touching or
displacing any solid thing, till their superficies come to meet;
whereby, I think, we have the clear idea of space without solidity.
For (not to go so far as annihilation of any particular body) I ask,
whether a man cannot have the idea of the motion of one single body
alone, without any other succeeding immediately into its place? I
think it is evident he can: the idea of motion in one body no more
including the idea of motion in another, than the idea of a square
figure in one body includes the idea of a square figure in another.
I do not ask, whether bodies do so exist, that the motion of one
body cannot really be without the motion of another. To determine this
either way, is to beg the question for or against a vacuum. But my
question is, whether one cannot have the idea of one body moved,
whilst others are at rest? And I think this no one will deny. If so,
then the place it deserted gives us the idea of pure space without
solidity; whereinto any other body may enter, without either
resistance or protrusion of anything. When the sucker in a pump is
drawn, the space it filled in the tube is certainly the same whether
any other body follows the motion of the sucker or not: nor does it
imply a contradiction that, upon the motion of one body, another
that is only contiguous to it should not follow it. The necessity of
such a motion is built only on the supposition that the world is full;
but not on the distinct ideas of space and solidity, which are as
different as resistance and not resistance, protrusion and not
protrusion. And that men have ideas of space without a body, their
very disputes about a vacuum plainly demonstrate, as is shown in
another place.
4. From hardness .
Solidity is hereby also differenced from hardness,
in that solidity consists in repletion, and so an utter exclusion of
other bodies out of the space it possesses: but hardness, in a firm
cohesion of the parts of matter, making up masses of a sensible
bulk, so that the whole does not easily change its figure. And indeed,
hard and soft are names that we give to things only in relation to the
constitutions of our own bodies; that being generally called hard by
us, which will put us to pain sooner than change figure by the
pressure of any part of our bodies; and that, on the contrary, soft,
which changes the situation of its parts upon an easy and unpainful
touch.
But this difficulty of changing the situation of the sensible parts amongst themselves, or of the figure of the whole, gives no more solidity to the hardest body in the world than to the softest; nor is an adamant one jot more solid than water. For, though the two flat sides of two pieces of marble will more easily approach each other, between which there is nothing but water or air, than if there be a diamond between them; yet it is not that the parts of the diamond are more solid than those of water, or resist more; but because the parts of water, being more easily separable from each other, they will, by a side motion, be more easily removed, and give way to the approach of the two pieces of marble. But if they could be kept from making place by that side motion, they would eternally hinder the approach of these two pieces of marble, as much as the diamond; and it would be as impossible by any force to surmount their resistance, as to surmount the resistance of the parts of a diamond. The softest body in the world will as invincibly resist the coming together of any other two bodies, if it be not put out of the way, but remain between them, as the hardest that can be found or imagined. He that shall fill a yielding soft body well with air or water, will quickly find its resistance. And he that thinks that nothing but bodies that are hard can keep his hands from approaching one another, may be pleased to make a trial, with the air inclosed in a football. The experiment, I have been told, was made at Florence, with a hollow globe of gold filled with water, and exactly closed; which further shows the solidity of so soft a body as water. For the golden globe thus filled, being put into a press, which was driven by the extreme force of screws, the water made itself way through the pores of that very close metal, and finding no room for a nearer approach of its particles within, got to the outside, where it rose like a dew, and so fell in drops, before the sides of the globe could be made to yield to the violent compression of the engine that squeezed it.
5. On solidity depend impulse, resistance, and protrusion .
6. What solidity is .
If any one ask me, What this solidity is, I
send him to his senses to inform him. Let him put a flint or a
football between his hands, and then endeavour to join them, and he
will know. If he thinks this not a sufficient explication of solidity,
what it is, and wherein it consists; I promise to tell him what it is,
and wherein it consists, when he tells me what thinking is, or wherein
it consists; or explains to me what extension or motion is, which
perhaps seems much easier. The simple ideas we have, are such as
experience teaches them us; but if, beyond that, we endeavour by words
to make them clearer in the mind, we shall succeed no better than if
we went about to clear up the darkness of a blind man's mind by
talking; and to discourse into him the ideas of light and colours. The
reason of this I shall show in another place.