Further, it can be shown that the repulsive power of heat in the solar system has already received the attention of scientists, especially in France. This will be seen more fully when we come to deal with the phenomena of comets' tails. One remarkable feature about comets' tails is, that they are always directed away from the sun, and various hypotheses have been advanced to account for that fact. Among them is the hypothesis of M. Faye, in which he assumes that there is a repulsive force which has its origin in the heat of the sun. This repulsive force is not propagated instantaneously, but the velocity of propagation is the same as that of a ray of light. By means of this repulsive power due to the heat of the sun, M. Faye explains how it is that the tails of comets are always turned away from the sun. Here, then, we have an indication of the existence of this repulsive force of heat which we are considering--a repulsive power which finds its source in the aetherial waves, which give rise to the phenomena of Heat, and to which we must look for the ultimate source of that repulsive power or Centrifugal Force which is to form the complementary power to the attractive force of Gravitation. [Footnote 12: _Lectures on Scientific Subjects._] ART. 65. _Direction of Ray of Heat._--The question as to the path which a ray of heat takes may best be attacked by finding out what is the path which a ray of light takes in its progress through the Aether. When we come to deal with light, we shall find that it has been experimentally proved that the path of a ray of light is that of a straight line through space; so that if we have any body emitting light, the rays of light will proceed from that body in straight lines, with decreasing intensity, according to the law of inverse squares, the same as Gravitation.