It can readily be shown, that wherever there is light there is heat. For example, the radiant heat from the sun proceeds through space along with the light from the sun, and when one set of waves, the light waves for instance, are intercepted, the heat waves are also intercepted. Or, to take another illustration, when the sun is eclipsed, we feel the sun's heat as long as any portion of the sun is visible, but as soon as the sun is totally eclipsed, then the light waves disappear, and with it the heat waves. From this we can readily see, that not only do the heat and light waves from the sun proceed in the same straight line, but that they also travel at the same rate through space, at the rate of 186,000 miles per second. Then again the common lens, which is so familiar to every one, will prove the same fact by concentrating the rays of light to a focus, and by so doing will produce sufficient heat to burn a piece of paper, or even set fire to wood. If, therefore, the path of a ray of light be that of a straight line, proceeding from the luminous or lighted body, and the path of a ray of heat coincides with the path of a ray of light, the path of the ray of heat must also be in the direction of a straight line from the heated or luminous body, which, as we shall see in a subsequent article, also decreases in intensity according to the law of inverse squares the same as Gravitation Attraction. Professor Tyndall, on the direction of a ray of heat,[13] states his opinion on the matter as follows: "A wave of Aether starting from a radiant point in all directions in a uniform medium constitutes a spherical shell, which expands with the velocity of light or of radiant heat. A ray of light or a ray of heat is a line perpendicular to the wave, and in the case here supposed, the rays would be the radii of the spherical shell." From this it can be seen that a ray of light or heat corresponds to what is known as the radius vector of a circle (Art. 20), and therefore a ray of light and heat takes exactly the same path through space (if we consider the sun as the source of the light and heat) as the path of the attractive power of Gravitation. Collecting, therefore, our results from the preceding articles of this chapter, we learn that heat is due to vibrating wave motion of the Aether, and that that motion is a motion which is always directed from the central body which is the source of the heat; and further, that this motion amounts to a repulsive motion acting in an opposite direction to the attractive power of gravity or to the centripetal force of Gravitation. What is more remarkable still, the path of a ray of heat corresponds with, and takes up exactly the same direction through space, whether it be atomic space, solar space, or interstellar space, as the attractive force of Gravitation.