Many processes of art, like the operations of nature, are dependent on heat. By this agent, the most obdurate masses soften like wax, and yield to the forms which are demanded by our wants and our tastes; and compounds, knit together by stubborn affinities, are resolved by it into their original elements. The baron Von Tchivanhausen constructed a burning mirror in 1687, five feet three inches in breadth, and reflecting the solar rays with extraordinary power. When exposed to its force, wood took fire, and continued to burn, notwithstanding a most violent wind; and water, contained in an earthen vessel, quickly boiled, so that eggs were cooked, and the liquid soon after evaporated. Copper and silver were fused in a few minutes, and slate was transformed into a kind of black glass, which, when held by a pair of pincers, could be drawn out into filaments. This mirror afterwards came into the possession of the king of France, and was kept in the Jardins du Roi. Other mirrors have been formed of different substances. At the Polytechnic Institution, some years ago, there were two metallic discs placed at the extreme ends of the great hall, and when a vessel of burning coals was held in the focus of one, and a piece of meat in the focus of the other, the latter was cooked with marvellous rapidity by a simple and apparently an unimportant instrument. The blow-pipe has immense power. Two volumes of hydrogen, and one of oxygen gas, when pure, form a mixture which produces in this instrument intense heat, and most substances may be fused by the flame. In the experiments of Dr. E. Clarke, lime, strontion, and alumine, yielded to its powers. The alkalis were fused and volatilized almost the instant they came into contact with the flame: and rock crystal became a transparent glass full of bubbles. Opal changed into a pearly white enamel, and flint into one that was frothy. Blue sapphire was melted; and Peruvian enamel changed into a transparent and colourless glass. Lapis lazuli fused into transparent glass, with a slight tinge of green. Iceland spar, next in difficulty, as to fusion, to its native magnesia, melted at last into a limpid glass, giving out an amethyst-coloured flame. Diamond first became opaque, and was then gradually volatilized. Gold, mixed with borax as a flux, was fused; platina wire melted the instant it was brought into contact with the flame, and ran down in drops; brass wire burned with a green flame; and iron wire with brilliant sparks.