There is some prospect of the luminosity excited in a vacuum tube by the alternating currents from a dynamo or an induction coil becoming an illuminant. Crookes has obtained exquisitely beautiful glows by the phosphorescence of gems and other minerals in a vacuum bulb like that shown in figure 69, where A and B are the metal electrodes on the outside of the glass. A heap of diamonds from various countries emit red, orange, yellow, green, and blue rays. Ruby, sapphire, and emerald give a deep red, crimson, or lilac phosphorescence, and sulphate of zinc a magnificent green glow. Tesla has also shown that vacuum bulbs can be lit inside without any outside connection with the current, by means of an apparatus like that shown in figure 70, where D is an alternating dynamo, C a condenser, P S the primary and secondary coils of a sparking transformer, T T two metal sheets or plates, and SB the exhausted bulbs. The alternating or see-saw current in this case charges the condenser and excites the primary coil P, while the induced current in the secondary coil 5 charges the terminal plates T T. So long as the bulbs or tubes are kept within the space between the plates, they are filled with a soft radiance, and it is easy to see that if these plates covered the opposite walls of a room, the vacuum lamps would yield a light in any part of it. Electric heating bids fair to become almost as important as electric illumination. When the arc was first discovered it was noticed that platinum, gold, quartz, ruby, and diamond--in fine, the most refractory minerals--were melted in it, and ran like wax. Ores and salts of the metals were also vapourised, and it was clear that a powerful engine of research had been placed in the hands of the chemist. As a matter of fact, the temperature of the carbons in the arc is comparable to that of the Sun. It measures 5000 to 10,000 degrees Fahrenheit, and is the highest artificial heat known. Sir William Siemens was among the first to make an electric furnace heated by the arc, which fused and vapourised metallic ores, so that the metal could be extracted from them. Aluminium, chromium, and other valuable metals are now smelted by its means, and rough brilliants such as those found in diamond mines and meteoric stones have been crystallised from the fumes of carbon, like hoar frost in a cold mist. The electric arc is also applied to the welding of wires, boiler plates, rails, and other metal work, by heating the parts to be joined and fusing them together.