The Nature of Solids and Fluids

53. The use of these rules is difficult because each body is touched by many at the same time

In the world, there are:

• no bodies that are so isolated from all others
• hardly any perfectly hard bodies around us

And so it is much more difficult to devise a calculation to determine how much the motion of each body is altered due to the encounters with others.

This is because we must consider all the factors that surround it. These have very different effects depending on whether they are hard or fluid.

Therefore, the diversity in this regard must be investigated.

54. Which bodies are hard, and which are fluids.

The particles of fluids easily move away from their positions. Therefore, they do not resist when our hands move towards them.

On the contrary, the particles of solids adhere to each other so strongly. They cannot be separated without a force sufficient to overcome their cohesion.

Bodies that are already in motion allow other bodies to fill the places that they have just left.

But bodies at rest can only be displaced from their positions through some force.

Hence:

• fluids are bodies that are divided into many small particles, agitated by mutually different motions.
• solids are bodies whose individual particles all remain at rest relative to each other

55. The particles of solid bodies are joined together by no other glue than their own stillness

We cannot conceive of any glue that would bind the particles of solid bodies more firmly than their own stillness.

What could this glue be?

These particles are already substances. They cannot be joined by another new substance. Therefore, they are joined by themselves.

They differ from other known substances only by their stillness. This stillness can oppose the motion that would separate these particles.