Another interesting vapour lamp may be mentioned briefly. This has a highly exhausted glass tube containing neon, a rare gas discovered by Sir William Ramsay. The light of this lamp contains no blue rays, and it is of a striking red colour. Neon lamps are used chiefly for advertising purposes, and they are most effective for illuminated designs and announcements, the peculiar and distinctive colour of the light attracting the eye at once. An electric current meets with some resistance in passing through any substance, and if the substance is a bad conductor the resistance is very great. As the current forces its way through the resistance, heat is produced, and a very thin wire, which offers a high resistance, may be raised to a white heat by an electric current, and it then glows with a brilliant light. This fact forms the basis of the electric incandescent or glow lamp. In the year 1878, Thomas A. Edison set himself the task of producing a perfect electric incandescent lamp, which should be capable of superseding gas for household and other interior lighting. The first and the greatest difficulty was that of finding a substance which could be formed into a fine filament, and which could be kept in a state of incandescence without melting or burning away. Platinum was first chosen, on account of its very high melting-point, and the fact that it was not acted upon by the gases of the air. Edison’s earliest lamps consisted of a piece of very thin platinum wire in the shape of a spiral, and enclosed in a glass bulb from which the air was exhausted. The ends of the spiral were connected to outside wires sealed into the bulb. It was found, however, that keeping platinum continuously at a high temperature caused it to disintegrate slowly, so that the lamps had only a short life. Fine threads or filaments of carbon were then tried, and found to be much more durable, besides being a great deal cheaper. The carbon filament lamp quickly became a commercial success, and up to quite recent years it was the only form of electric incandescent lamp in general use. In 1903 a German scientist, Dr. Auer von Welsbach, of incandescent gas mantle fame, produced an electric lamp in which the filament was made of the metal osmium, and this was followed by a lamp using the metal tantalum for the filament, the invention of Siemens and Halske. For a while the tantalum lamp was very successful, but more recently it has been superseded in popularity by lamps having a filament of the metal tungsten. The success of these lamps has caused the carbon lamp to decline in favour. The metal filaments become incandescent much more easily than the carbon filament, and for the same candle-power the metal filament lamp consumes much less current than the carbon lamp.