When man was endowed with fire he received the gift without knowing whence it came or what it meant. Greek mythology, with its deep insight into unseen things, presents us Helios, the sun-god, as a friend of Hephæstus, the god of fire. We moderns express the same truth differently. We say that all fire on earth is ultimately derived from the sun. Archimedes kindled fires――preferably in Roman ships――by means of concave mirrors which condensed the heat of the sun to a focus. The fuel for the fires was also provided by the heat of the sun, absorbed by the chlorophyll of the leaves in the forest and used to build up combustible wood and other products. The firing of the wood reversed the process of accumulation of solar energy and provided light and heat by day and by night, independently of the sun, the original giver. For a hundred million years already had the sun shone on a habitable earth, pouring out its light and heat and nourishing the luxuriant vegetation which covered most of the globe. For untold ages had the plants stored up the sunlight for some unknown end. That end became manifest when Prometheus kindled the first fire and made a new realm accessible to man. Till then, mankind had undergone the annual and daily vicissitudes of heat and cold due to the days and the seasons, but he had not controlled them. When he acquired the mastery of fire, he was enabled to wander north and south and take with him in his brazier the accustomed heat of the tropics. But there was more to come. The invention of pottery――another great epoch――enabled him to create a solar furnace on a small scale. It enabled him to create a miniature world where the heat was much greater than in his own world. He soon found that plants and animals, passed through this new world, became richer and more palatable, and in many ways better adapted to his digestive system. It must have seemed highly “unnatural” at first to accelerate the ripening of fruits and predigest animal food by plunging them into water in which no living thing could survive, but the departure from the habits of the animal world, once begun, was never arrested, and it is still proceeding in our own day. Our remote progenitors must have looked upon the guardians of their fires with much the same awe as that which mediæval people felt for sorcerers and alchemists, and with rather more justification. For man had everything to learn about the new power placed in his hands. He was beginning to find his way about a new world in which the ordinary laws of Nature were suspended. Water was no longer cold and wet in that world. It was hot as the sun and thin as smoke. It no longer flowed but rose in the air. Salt rapidly became invisible in it, but could as rapidly be recovered by adding more fire. There was as much to learn about fire as there was about X-rays in 1895 and about “atmospherics” to-day.