{175} The variation of our supply of light from the sun is the only explanation we have of the great alternations of heat and cold which have been so extensive as, at one period, to have clothed high northern latitudes, such as Greenland and Siberia, with a more than tropical luxuriance of vegetation, and, at another time, to have buried vast tracts of Europe and America, now enjoying a genial climate, under vast glaciers and mountains of ice. Again, light, so far from being solely derived from the sun, exists in, and can be educed from almost any known substance. The metallic bases of most earths and alkalies are capable of emitting light in suitable electrical conditions, and a brilliant flame can be produced by the combustion even of water. All the metals can be made to flash forth lightnings under suitable electric and magnetic excitements; and the crystals of several rocks give out light during the process of crystallization. Thousands of miles of the earth's surface must once have presented the lurid glow of a vast furnace of melted granite. Even at a far later period of its history, it may have shone with a luster little inferior to that of the sun; for lime, of which unknown thousands of miles of its strata consist, when subject to a heat much less than that produced by contact with melted granite or lava, emits a brilliant, white light of such intensity that the eye cannot support its luster. (_See Turner's Chemistry, Sec_. 160.) As is well known, the moon is a dark, opaque body, therefore the copper color of the moon, {176} during a total eclipse, when the dark side of the earth is turned towards the moon, shows us that the earth, even now, is a source of light. That God could command the light to shine out of darkness, and convert the very ocean into a magnificent illumination, the following fact clearly proves: