Electric light is also produced by sending a discharge through vacuum tubes like those of Geissler; and the varied colours thus produced are exceedingly pretty. Phosphorescent substances, too, such as the sulphide of barium, or the platino-barium cyanide, become highly luminous when inclosed in a tube and traversed by the electric current. Besides the voltaic arc, we have now, however, another kind of electric light—namely, the incandescence which is produced by sending the current through a very slender filament of platinum wire or carbon fibre inclosed in a glass bulb exhausted of air. Such are the lamps of Swan, Edison, and others. These lamps have also their curious features. The temperature of the filament is of course much lower than the temperature of the arc. It is only about eighteen hundred degrees Centigrade, for if it were higher, the delicate filament would be dissipated into vapour which would condense like smoke on the cool glass. With a platinum filament, the metal would ‘silver’ the interior of the bulb. Curiously enough, when the copper ‘electrodes’ or wires conveying the current inside the bulb to the filament of an Edison lamp are accidentally dissipated by excess of current, the carbon thread seems to shelter the glass from the copper shower, for Dr J. Fleming has observed that there is always a blank line on the glass opposite the filament, while all the rest is coated with a film of copper. When the carbon itself is dissipated, this blank line is not seen, and the whole interior of the bulb appears to be smoked. According to Dr Fleming, this means that the molecules of copper move in straight lines in the vacuum. During the ordinary action of one of these lamps there is believed to be a kind of molecular bombardment between the two sides of the carbon filament, which is usually bent into a loop. This battery of atoms in time disintegrates the filament near its junction with the wires where it is severest, and a patent has recently been taken out by Mr Brush, the well-known inventor, for the insertion of a mica screen between the legs of the filament to shield them from the pellets. The spectrum of the voltaic arc consists of the continuous ribbon spectrum of the white-hot solid carbons, and certain bright lines due to the glowing vapours of the arc. The light is rich in the blue or actinic rays so productive of chemical action, and hence it is, perhaps, that Dr Siemens found it so effective in forcing fruit and flowers by night in lieu of the sun. It helps the development of chlorophyl; and perhaps the electricity itself has also something to do with assisting growth, apart from the light, for several French experimenters have found that electrified soil and air seem to foster plants better than unelectrified. It is remarkable, too, that young bamboo shoots grow very rapidly after the thunderstorms which usher in the Indian monsoons.