The practical application of natural gas, as an article of fuel, to the purpose of manufacturing glass, iron, and steel, promises to work a revolution in the industrial interests of America--promises to work a revolution; for notwithstanding the fact that, in many of the largest iron, steel, and glass factories in Pittsburg and its vicinity, natural gas has already been substituted for coal, the managers of some such works are shy of the new fuel, mainly for two reasons: 1. They doubt the continuity and regularity of its supply. 2. They do not deem the difference between the price of natural gas and coal sufficient as yet to justify the expenditure involved in the furnace changes necessary to the substitution of the one for the other. These two objections will doubtless disappear with additional experience in the production and regulation of the gas supply, and with enlarged competition among the companies engaging in its transmission from the wells to the works. At present the use of natural gas as a substitute for coal in the manufacture of glass, iron, and steel is in its infancy. Natural gas is as ancient as the universe. It was known to man in prehistoric times, we must suppose, for the very earliest historical reference to the Magi of Asia records them as worshiping the eternal fires which then blazed, and still blaze, in the fissures of the mountain heights overlooking the Caspian Sea. Those records appertain to a period at least 600 years before the birth of Christ; but the Magi must have lived and worshiped long anterior to that time. Zoroaster, reputed founder of the Parsee sect, is placed contemporary with the prophet Daniel, from 2,500 to 600 B.C.; and, although Daniel has been doubted, and Zoroaster may never have seen the light, the fissures of the Caucasus have been flaming since the earliest authentic records. The Parsees (Persians) did not originally worship fire. They believed in two great powers--the Spirit of Light, or Good, and the Spirit of Darkness, or Evil. Subsequent to Zoroaster, when the Persian empire rose to its greatest power and importance, overspreading the west to the shores of the Caspian and beyond, the tribes of the Caucasus suffered political subjugation; but the creed of the Magi, founded upon the eternal flame-altars of the mountains, proved sufficiently vigorous to transform the Parseeism of the conquerors to the fire worship of the conquered. About the beginning of the seventh century of the Christian era, the Grecian Emperor Heraclius overturned the fire altars of the Magi at Baku, the chief city on the Caspian, but the fire worshipers were not expelled from the Caucasus until the Mohammedans subjugated the Persian Empire, when they were driven into the Rangoon, on the Irrawaddy, in India, one of the most noted petroleum producing districts of the world.