Several French scientists have suggested that the repulsive force was due to the heat of the sun. M. Roche was one of those who stated that the phenomena of cometary tails was due to the repulsive power of heat, which found its origin in the heat of the sun. M. Faye, another French scientist, states that the repulsive force had its origin in the heat of the sun. By a series of experiments he demonstrated that there was a repulsive power in all heat waves, which gave his theory that experimental support that any theory must possess to make it permanent. Now in Art. 63 it was shown that heat does possess a repulsive power, but that that power is rather due to the electro-magnetic Aether whose vibrations produce the heat waves, than to the repulsion of heat; so that, indirectly, the assumption of both these French scientists, that the repulsive power of heat gave rise to the tails of comets, is correct. Then again it has been suggested that the repulsive power is produced by the pressure of the light waves. Professor Lebedew suggested this after he had experimentally proved that light waves did possess a repulsive power (_Annalen der Physik_, November 1901). It can easily be seen, as pointed out in Art. 70, that, inasmuch as light is due to the vibrations of the Aether, they too possess this repulsive power, and therefore Professor Lebedew's suggestion as to the nature of the repulsive power is correct, as the real centrifugal force is really due to an aetherial pressure. Whether, therefore, we consider it from the standpoint of heat, or light or electricity, it ultimately resolves itself into the same aetherial medium which is at once the common source of all these forces. Again, it has been suggested that the repulsive power is electrical or electro-magnetic, and this view is receiving more support than either of the others from modern scientists. Herschel suggested that the repulsive power was electrical, while Bredichin has worked out a very careful theory as to the effect of electrical repulsion upon different elements that are found in the comets' tails, with a view to explain the different shapes of the tails. But whether the force is looked at from the standpoint of heat, light or electricity, it ultimately resolves itself into the motions of the Aether, which gives rise by its different vibrations and motions to all the three forms of energy referred to.