we have nothing to fear. But the same forces of air and water, and of the volcanic interior, which produced former geological revolutions, and buried one series of living forms after another, still act upon the earth's crust. They, rather than those distant cosmical changes of which we have spoken, will end the human race, and perhaps compel us to make way for new and more complete forms of life, as the lizard and the mammoth have given way to us and our contemporaries. "Grand, however, and marvelous as are these questions regarding the physical constitution of the sun, they are but a portion of the wonders connected with our luminary. His relationship to life is yet to be referred to. The earth's atmosphere contains carbonic acid, and the earth's surface bears living plants; the former is the nutriment of the latter. The plant seizes the combined carbon and oxygen and tears them asunder, storing the carbon and letting the oxygen go free. By no special force, different in quality from other forces, do plants exercise this power--the real magician here is the sun. We have seen how heat is consumed in forcing asunder the atoms and molecules of solids and liquids, converting itself into potential energy, which reappears as heat when the attractions of the separated atoms are again allowed to come into play. Precisely the same considerations which we then applied to heat we have now to apply to light; for it is at the expense of the solar light that the decomposition of the carbonic acid is effected. Without the sun the reduction cannot take place, and an amount of sunlight is consumed exactly equivalent to the molecular work accomplished. Thus trees are formed, thus meadows grow, thus the flowers bloom. Let the rays fall upon the surface of sand, the sand is heated, and finally radiates away as much as it receives; let the same rays fall upon a forest, the quantity of heat given back is less than that received, for the energy of a portion of the sunbeams is invested in building up the trees. I have here a bundle of cotton which I ignite; it bursts into flame, and yields a definite amount of heat; precisely that amount of heat was abstracted from the sun in order to form that bit of cotton. This is a representative case--every tree, plant, and flower, grows and flourishes by the grace and bounty of the sun.