Hardness and Liquidity

There is an infinity of diverse motions that endure perpetually in the world. The greatest of these bring about the days, months, and years.

• The vapors of the earth never cease to rise to the clouds and descend from them.
• The air is forever agitated by the winds
• The sea is never at rest
• Springs and rivers flow ceaselessly
• The strongest buildings finally fall into decay
• Plants and animals are always either growing or decaying
• In short, all is changing.

Whence I know clearly that it is not in the flame alone that there are a number of small parts never ceasing to move, but that there are also such parts in every other body, even though their actions are not as violent and they cannot, due to their smallness, be perceived by any of our senses.

I think they began to move as soon as the world began to exist.

The virtue or power in a body to move itself can well pass wholly or partially to another body and thus no longer be in the first. But it cannot no longer exist in the world.

The Aether

The aether is the first mover which rolls around the universe at an incomprehensible speed. It is the origin and source of all the other motions found therein.

Notice first the difference between hard and liquid bodies.

Every body can be divided into extremely small particles*.

• There are several millions in the smallest grain of sand that our eyes can perceive.
• if two of these small parts are touching one another, some force is necessary to separate them, however small it may be.
  • Twice as much force is necessary to separate two of them than to separate one of them.
  • 1,000 times as much force is necessary to separate 1,0000 of them.

Thus, a sensible force is necessary to separate several millions of them all at once.

But if half of a body is moving to the left and the other half to the right, then less force is needed to separate them compared to if they were not moving.

• No force at all is needed if their motions separate themselves from each other.

Solid bodies are harder to separate than liquid bodies. Thus, the hardest body imaginable is one where its parts have no space remaining between them, and no movement in its parts.

The most liquid body has all of its smallest parts moving away from one another in the most diverse ways and as quickly as possible, even though they always:

• touch each other on all sides and
• arrange themselves in as small a space as if they were without motion.

Every body approaches these extremes, as its parts are more or less in the act of moving away from one another.

Physical Fire

All the parts of flame are perpetually agitated. Its parts are liquid and it also renders most other bodies liquid.

• When it melts metals, it acts with no different power than when it burns wood.

Rather, because the parts of metals are just about all equal, the flame cannot move one part without moving the other, and hence it forms completely liquid bodies from them.

By contrast, the parts of wood are unequal in such a way that the flame can separate the smaller of them and render them liquid (i.e. cause them to fly away in smoke) without agitating the larger parts.

Physical Air

After flame, air is the most liquid. We see its parts move separately from one another.

For, if you take the effort to watch those small bodies that are commonly called “atoms” and that appear in rays of sunlight, you will see them flutter about incessantly here and there in a myriad of different ways, even when there is no wind stirring them up.

One can also experience the same sort of thing in all the grosser liquids if one mixes them together in different colors, in order better to distinguish their motions. Finally, the phenomenon appears very clearly in acids[13] when they move and separate the parts of some metal.

It is only the motion of the parts of flame that cause it to burn and make it liquid.

Why is it that the motion of the parts of air, which also make it extremely liquid, do not burn it?

We must take into account both the speed of motion and the size of the parts. The smaller ones make the more liquid bodies. But the larger ones have more force to burn and act on other bodies.

A part is something that is joined together and is not in the act of separation, even though the smallest parts could easily be divided into many other smaller ones.

• Thus, a grain of sand, a stone, a rock, the whole earth is a single part as long as we refer to a completely simple and equal motion.[14]
• Air that has large parts compared to the others will move very slowly.
• Air that has small parts compared to others will move more quickly

Fire can burn because its largest parts are as large as those of air but move more quickly.

• Its smaller parts penetrate many bodies of which the pores are so narrow that even air cannot enter.

As far as the larger parts are concerned (or the equally large parts in greater number), one sees clearly how air alone does not suffice to nourish flame. The violence of their action is enough to show us that they move more quickly.

It is the largest of these parts that have the power to burn, and not the others, is apparent from the fact that the flame that issues from brandy, or from other very subtle bodies, hardly burns at all, while on the contrary that which is engendered in hard and heavy bodies is very hot.