Both coal and oil-gas develop a great amount of heat in proportion to the light they yield. The hydrogen they contain in large quantities burns, when pure, with an almost invisible flame, but more hotly than any other known gas. The particles of carbon also present in the flame are heated to whiteness by the hydrogen, but they are not sufficient in number to convert more than a fraction of the heat into light. A German, Auer von Welsbach, conceived the idea of suspending round the flame a circular “mantle” of woven cotton steeped in a solution of certain rare earths (_e.g._ lanthanum, yttrium, zirconium), to arrest the heat and compel it to produce bright incandescence in the arresting substance. With the same gas consumption a Welsbach burner yields seven or more times the light of an ordinary batswing burner. The light itself is also of a more pleasant description, being well supplied with the blue rays of the spectrum. The mantle is used with other systems than the ordinary gas-jet. Recently two methods of illumination have been introduced in which the source of illumination is supplied under pressure. The high-pressure incandescent gas installations of Mr. William Sugg supply gas to burners at five or six times the ordinary pressure of the mains. The effect is to pulverise the gas as it issues from the nozzle of the burners, and, by rendering it more inflammable, to increase its heating power until the surrounding mantle glows with a very brilliant and white light of great penetration. Gas is forced through the pipes connected with the lamps by hydraulic rams working gas-pumps, which alternately suck in and expel the gas under a pressure of twelve inches (_i.e._ a pressure sufficient to maintain a column of water twelve inches high). The gas under this pressure passes into a cylinder of a capacity considerably greater than the capacity of the pumps. This cylinder neutralises the shock of the rams, when the stroke changes from up-to downstroke, and _vice versâ_. On the top of the cylinder is fixed a governor consisting of a strong leathern gas-holder, which has a stroke of about three inches, and actuates a lever which opens and closes the valve through which the supply of water to the rams flows, and reduces the flow of the water when it exceeds ten or twelve inches pressure, according to circumstances. The gas-holder of the governor is lifted by the pressure of the gas in the cylinder, which passes through a small opening from the cylinder to the governor so as not to cause any sudden rise or fall of the gas-holder. By this means a nearly constant pressure is maintained; and from the outlet of the cylinder the gas passes to another governor sufficient to supply the number of lights the apparatus is designed for, and to maintain the pressure without variation whether all or a few lamps are in action. For very large installations steam is used.