**Features of Fatio's Theory**

- Fatio's pyramid (Problem I)

Fatio assumed that the universe is filled by minute particles, which are moving indiscriminately with very high speed and rectilinearly in all directions. To illustrate his thoughts he used the following example: Suppose an object *C*, on which an infinite small plane zz and a sphere centered about *zz* is drawn. Into this sphere Fatio placed the pyramid *PzzQ*, in which some particles are streaming in the direction of *zz* and also some particles, which were already reflected by *C* and therefore depart from *zz*. Fatio proposed that the mean velocity of the reflected particles is lower and therefore their momentum is weaker than that of the incident particles. The result is *one stream*, which pushes all bodies in the direction of *zz*. So on one hand the speed of the stream remains constant, but on the other hand at larger proximity to *zz* the density of the stream increases and therefore its intensity is proportional to 1/r2. And because one can draw an infinite number of such pyramids around *C*, the proportionality applies to the entire range around *C*.

- Reduced speed

In order to justify the assumption, that the particles are traveling after their reflection with diminished velocities, Fatio stated the following assumptions:

- Either ordinary matter, or the gravific particles, or both are inelastic, or
- the impacts are fully elastic, but the particles are not absolutely hard, and therefore are in a state of vibration after the impact, and/or
- due to friction the particles begin to rotate after their impacts.

These passages are the most incomprehensible parts of Fatio's theory, because he never clearly decided which sort of collision he actually preferred. However, in the last version of his theory in 1742 he shortened the related passages and ascribed "perfect elasticity or spring force" to the particles and on the other hand "imperfect elasticity" to gross matter, therefore the particles would be reflected with diminished velocities. Additionally, Fatio faced another problem: What is happening if the particles collide with each other? Inelastic collisions would lead to a steady decrease of the particle speed and therefore a decrease of the gravitational force. To avoid this problem, Fatio supposed that the diameter of the particles is very small compared to their mutual distance, so their interactions are very rare.

- Condensation

Fatio thought for a long time that, since corpuscles approach material bodies at a higher speed than they recede from them (after reflection), there would be a progressive accumulation of corpuscles near material bodies (an effect which he called "condensation"). However, he later realized that although the incoming corpuscles are quicker, they are spaced further apart than are the reflected corpuscles, so the inward and outward flow rates are the same. Hence there is no secular accumulation of corpuscles, i.e., the density of the reflected corpuscles remains constant (assuming that they are small enough that no noticeably greater rate of self-collision occurs near the massive body). More importantly, Fatio noted that, by increasing both the velocity and the elasticity of the corpuscles, the difference between the speeds of the incoming and reflected corpuscles (and hence the difference in densities) can be made arbitrarily small while still maintaining the same effective gravitational force.

- Porosity of gross matter

In order to ensure mass proportionality, Fatio assumed that gross matter is extremely permeable to the flux of corpuscles. He sketched 3 models to justify this assumption:

- He assumed that matter is an accumulation of small "balls" whereby their diameter compared with their distance among themselves is "infinitely" small. But he rejected this proposal, because under this condition the bodies would approach each other and therefore would not remain stable.
- Then he assumed that the balls could be connected through bars or lines and would form some kind of crystal lattice. However, he rejected this model too - if several atoms are together, the gravific fluid is not able to penetrate this structure equally in all direction, and therefore mass proportionality is impossible.
- At the end Fatio also removed the balls and only left the lines or the net. By making them "infinitely" smaller than their distance among themselves, thereby a maximum penetration capacity could be achieved.

- Pressure force of the particles (Problem II)

Already in 1690 Fatio assumed, that the "push force" exerted by the particles on a plain surface is the sixth part of the force, which would be produced if all particles are lined up normal to the surface. Fatio now gave a proof of this proposal by determination of the force, which is exerted by the particles on a certain point zz. He derived the formula *p=ρv2zz/6*. This solution is very similar to the formula known in the kinetic theory of gases *p=ρv2/3*, which was found by Daniel Bernoulli in 1738. This was the first time that a solution analogous to the similar result in kinetic theory was pointed out - long *before* the basic concept of the latter theory was developed. However, Bernoulli's value is twice as large as Fatio's one, because according to Zehe, Fatio only calculated the value *mv* for the change of impulse after the collision, but not *2mv* and therefore got the wrong result. (His result is only correct in the case of totally inelastic collisions.) Fatio tried to use his solution not only for explaining gravitation, but for explaining the behaviour of gases as well. He tried to construct a thermometer, which should indicate the "state of motion" of the air molecules and therefore estimate the temperature. But Fatio (unlike Bernoulli) did not *identify* heat and the movements of the air particles - he used another fluid, which should be responsible for this effect. It is also unknown, whether Bernoulli was influenced by Fatio or not.

- Infinity (Problem III)

In this chapter Fatio examines the connections between the term *infinity* and its relations to his theory. Fatio often justified his considerations with the fact that different phenomena are "infinitely smaller or larger" than others and so many problems can be reduced to an undetectable value. For example the diameter of the bars is infinitely smaller than their distance to each other; or the speed of the particles is infinitely larger than those of gross matter; or the speed difference between reflected and non-reflected particles is infinitely small.

- Resistance of the medium (Problem IV)

This is the mathematically most complex part of Fatio's theory. There he tried to estimate the resistance of the particle streams for moving bodies. Supposing *u* is the velocity of gross matter, *v* is the velocity of the gravific particles and *ρ* the density of the medium. In the case *v << u* and *ρ = const.* Fatio stated that the resistance is *ρu2*. In the case *v >> u* and *ρ = const.* the resistance is *4/3ρuv*. Now, Newton stated that the lack of resistance to the orbital motion requires an extreme sparseness of any medium in space. So Fatio decreased the density of the medium and stated, that to maintain sufficient gravitational force this reduction must be compensated by changing v "*inverse proportional to the square root of the density*". This follows from Fatio's particle pressure, which is proportional to *ρv2*. According to Zehe, Fatio's attempt to increase v to a very high value would actually leave the resistance very small compared with gravity, because the resistance in Fatio's model is proportional to *ρuv* but gravity (i.e. the particle pressure) is proportional to *ρv2*.

Read more about this topic: Le Sage's Theory Of Gravitation, Fatio

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