The Nature Of Terrestrial Bodies

This first discourse will deal with the nature of terrestrial bodies in general, so that I can better explain in the following one the nature of exhalations and vapors.

The vapors rising from the seawater sometimes form salt above its surface.

Philosophers say that:

• such forms of these bodies are composed of elements by a perfect mixture
• bodies such as meteors are composed of elements by an imperfect mixture.

I will explain:

• the winds through the vapors through the air
• the clouds by the vapors gathering in some places
• the rain, hail, and snow by how those clouds dissolve

The particles of snow are shaped like small six-pointed stars very perfectly arranged. These were not observed by the ancients. But it nonetheless is one of Nature’s rarest wonders.

I will also explain the tempests, thunder, lightning, and the various fires or lights that ignite in the air.

But above all, I will explain the rainbow and its colors so that one can also understand the nature of all those found in other subjects.

I will add the cause of:

• halos around the stars
• the multiple suns or moons that sometimes appear together.

The knowledge of these things depends on the general principles of Nature, which have not yet been well explained.

Therefore, I will have to start with some suppositions, as I did in the Dioptrics.

Water, earth, air, and all other such bodies that surround us are composed of many small particles of various shapes and sizes. These have many gaps around them.

These gaps are not empty, but filled with this very subtle matter, through the mediation of which I have said above that the action of light is communicated.

Then, in particular, I suppose that the small parts that water is composed of are long, smooth, and slippery, like little eels, which, although they join and intertwine, never tie or hook together in such a way that they cannot be easily separated.

Conversely, almost all those of earth as well as of air, and most other bodies, have very irregular and uneven shapes.

They cannot be even slightly intertwined without catching and binding to one another, just as the various branches of shrubs that grow together in a hedge do.

When they bind in this way, they form hard bodies, like earth, wood, or similar substances.

Whereas, if they are simply placed one on top of the other, without being very much or at all intertwined, and if they are also so small that they can be moved and separated by the agitation of the subtle matter surrounding them, they must occupy a lot of space and form liquid bodies, very rarefied and very light, like oils or air.

The subtle matter fills the gaps between the parts of these bodies.

It always move back and forth very quickly, not exactly at the same speed everywhere and at all times. It commonly moves

• a little faster at sea level than at the upper atmosphere
• faster near the equator than near the poles
• in the same place faster in summer than in winter
• faster during the day than at night.

This is because light is just luminous bodies pushing this fire-aether in all directions around them in a straight line, as was said in the Dioptrics.

Therefore, the sun’s rays, both direct and reflected, agitates it:

• more during the day than at night
• more in summer than in winter
• more near the equator than near the poles
• more against the earth than towards the clouds.

It follows that this fire-aether is composed of various very small particles. The largest particle has always the most force, just as large bodies have more force than the smaller ones when they are equally shaken.

This means that the larger (less subtle) its particles are, the more it can agitate the particles of other bodies.

It is largest when and where it is most agitated such as:

• near the surface of the earth rather than towards the clouds
• near the equator rather than near the poles
• in summer rather than in winter
• during the day rather than at night.

This is because the larger parts have more force. They can move more easily towards places where the agitation is greater. This makes it easier for them to continue their movement.

Nevertheless, there are always many very small particles that slip among these larger ones.

All terrestrial bodies have many pores through which these smaller parts can pass. But several have pores so narrow or constricted that they do not accept the larger parts.

These usually feel the coldest when touched or even when approached.

Marble and metals feel colder than wood. It means that:

• their pores do not easily accept the less subtle particles of the fire-aether.
• the pores of ice accept the fire-aether even less easily than those of marble or metals, because it is even colder.

For I suppose here that for cold and heat, there is no need to conceive anything other than that

• Heat is the small parts of the bodies that we touch being agitated more strongly than usual.
• Cold is them being agitated less than usual.

This agitation can be caused by the small particles of the fire-aether or by any other cause.

This agitation transfers to the small filaments of our nerves that are the organs of touch.

The particles of hard bodies are like intertwined branches.

This fire-aether does not separate those particles in the same way that it does those of water and liquids.

But it nonetheless agitates and makes them tremble according to:

• how strong its movement is
• how large its particles are

This is like how the wind can agitate all the branches of shrubs that form a hedge without removing them from their places.

There is a proportion between:

• the force of this fire-aether
• the resistance of the particles of bodies

Ice becomes water then it is agitated and by larger fire-aether particles that are near the surface of the earth [at sea level]. These larger fire-aether particles then have the force to:

• agitate and move the ice particles separately from one another
• bend most of the small particles of the water among which it slips, thus making it liquid.

Water becomes ice when it becomes less agitated and the fire-aether becomes more subtle, such as those that are in the atmosphere, or during winter. The fire-aether then does not have enough force to bend and agitate the water particles.

This causes the water to remain confusedly joined and laid one on top of the other, thus composing a hard body as ice.

The difference between water and ice is like the difference between eels:

• floating in a fisherman’s boat full of holes through which the river water passes and agitates them, and
• the same eels dry and stiff with cold on the shore.

Water only freezes when the fire-aether between its particles is more subtle than usual.

It follows that the pores of such ice are arranged in such a way that they cannot accept the larger and less subtle fire-aether.

Thus, the ice is always very cold, even when kept until summer. It retains its hardness without gradually softening like wax, because the heat only penetrates inside as the surface becomes liquid.