The mean density, however, of each atomic or molecular atmosphere is regulated solely by the mass of the atom or planet, therefore the pressure exerted by one atom on another is proportionate to the mass of each atom, and to that extent is strictly in accordance with the law which governs the proportion of the forces between the two atoms or molecules. If, therefore, we have two atoms, A and B, of different masses, then it is true that while A exerts a pressure on B, which pressure takes the form of a repulsion, at the same time B exerts a pressure on A which is equal and opposite in its character and intensity, and in each case the pressure is due to the aetherial medium which surrounds each atom or molecule. When the atoms are equal in mass, then the resultant motion produced on each atom would be exactly equal, but when the masses vary, the resultant motion produced on each atom would vary also, though the momentum in each case would be exactly equal and opposite, as momentum is a compound term dependent partly upon the mass of the body concerned. In Art. 16 we saw that when this third law was applied to planetary phenomena, not only did the sun attract all the planets, but all the planets attracted the sun with equal and opposite forces, and the planets also attracted each other with equal and opposite forces. In the same way it can be proved, that as the sun repels all the planets by the pressure exerted by the aetherial centrifugal force on those planets, the planets repel the sun with an exactly equal and opposite force at their mean distances. In Newton's conception, however, of the third law of motion, there was simply mathematical data to deal with, by which the law was shown to apply to the planetary and stellar world. In the case, however, of the centrifugal motion, we have a definite physical medium, which by its motions produces the pressure on the planets or suns that exist in space, which pressure forms the physical centrifugal force that forms the counterpart of Gravitation Attraction. Let us look at this phase of the case in detail, and by so doing help to establish and confirm the physical existence of the force or motion referred to. We have learned from Chapter IV. that as Aether is gravitative, it surrounds all satellites and planets, suns and stars that exist in the universe. We have also learned from Art. 86 that Aether has an electro-magnetic basis, as mathematically proved by Maxwell and experimentally proved by Hertz. Thus we came to the conclusion that each satellite and planet, sun and star, was an electrified body (Art. 81), or an electro-magnet (Art. 88), possessing its own electric or electro-magnetic field.