are the source of all the forms of energy practically available, except that of the tides. It has been estimated that the heat received by the earth from the sun each year would melt a layer of ice over the entire globe a hundred feet in thickness. This represents energy equal to one horse-power for each fifty square feet of surface, and the heat which reaches the earth is only one twenty-two-hundred-millionth of the heat that leaves the sun. Notwithstanding this enormous expenditure of energy, Helmholtz and Thomson have shown that the nebular hypothesis, which supposes the solar system to have originally existed as a chaotic mass of widely separated gravitating particles, presents to us an adequate source for all the energy of the system. As the particles of the system rush together by their mutual attractions, heat is generated by their collision; and after they have collected into large masses, the condensation of these masses continues to generate heat.
237. Dissipation of Energy.—It has been seen that only a fraction of the energy of heat is available for transformation into other forms of energy, and that such transformation is possible only when a difference of temperature exists. Every conversion of other forms of energy into heat puts it in a form from which it can be only partially recovered. Every transfer of heat from one body to another, or from one part to another of the same body, tends to equalize temperatures, and to diminish the proportion of energy available for transformation. Such transfers of heat are continually taking place; and, so far as our present knowledge goes, there is a tendency toward an equality of temperature, or, in other words, a uniform molecular motion, throughout the universe. If this condition of things were reached, although the total amount of energy existing in the universe would remain unchanged, the possibility of transformation would be at an end, and all activity and change would cease. This is the doctrine of the dissipation of energy to which our limited knowledge of the operations of Nature leads us; but it must be remembered that our knowledge is very limited, and that there may be in Nature the means of restoring the differences upon which all activity depends.
