For a long time experience had impressed on our mechanicians that a working force cannot be produced from nothing; that it can only be taken from the stores which nature possesses; which are strictly limited and which cannot be increased at pleasure--whether it be taken from the rushing water or from the wind; whether from the layers of coal, or from men and from animals, which cannot work without the consumption of food. Modern physics has attempted to prove the universality of this experience, to show that it applies to the great whole of all natural processes, and is independent of the special interests of man. These have been generalised and comprehended in the all-ruling natural law of the _Conservation of Force_. No natural process, and no series of natural processes, can be found, however manifold may be the changes which take place among them, by which a motive force can be continuously produced without a corresponding consumption. Just as the human race finds on earth but a limited supply of motive forces, capable of producing work, which it can utilise but not increase, so also must this be the case in the great whole of nature. The universe has its definite store of force, which works in it under ever varying forms; is indestructible, not to be increased, everlasting and unchangeable like matter itself. It seems as if Goethe had an idea of this when he makes the earth-spirit speak of himself as the representative of natural force. In the currents of life, in the tempests of motion, In the fervour of art, in the fire, in the storm, Hither and thither, Over and under, Wend I and wander. Birth and the grave, Limitless ocean, Where the restless wave Undulates ever Under and over, Their seething strife Heaving and weaving The changes of life. At the whirling loom of time unawed, I work the living mantle of God. Let us return to the special question which concerns us here: Whence does the sun derive this enormous store of force which it sends out?