Within the past few months there has been presented to the public a system of electric lighting which, in point of interest and results, bids fair to equal any improvement in lighting methods put forth in the past few years. This invention, known as the Cooper-Hewitt vapor lamp, was exhibited to the public first at a meeting of the American Electrical Engineers last spring. In construction this lamp differs very strikingly from the familiar incandescent lamp, and its principle of operation is also different from the arc light. The lamp has not yet been developed to a commercial stage, but the experimental period is so far successfully passed as to warrant the assertion that it will be on the market within a comparatively short time. In appearance the lamp consists of a glass tube, nearly an inch in diameter, and about three feet long. The ends of the tube are in some models expanded into bulbs, and in each end of the tube or in each bulb is inserted a single leading-in wire, terminating at upper end in a metal cup-shaped electrode, preferably iron, while on the lower end of the lamp is contained a small quantity of mercury, which is in metallic contact with the leading-in wire at this point. The air is exhausted from this lamp by a process similar to that used in the manufacture of incandescent lamps of the ordinary type. As the air is exhausted, the tube becomes filled with mercury vapor, which at a certain stage or vacuum becomes conducting to a current of relatively low pressure or voltage. In the samples so far exhibited, when connected to a 110 or 220 circuit, this current is first started through the lamp from one end to the other by a special device, when it continues to flow uninterruptedly, and gives rise to an intense light throughout the entire length of the tube, so long as the current is applied to the lamp. The results are practically the same, whether direct or alternating currents are used, although which current is the most satisfactory in commercial practice is not yet announced. The intense light generated by the passage of the current through the rarified mercury vapor is not caused by an arc between the terminals of the lamp, but is a true form of conduction. There is no intense heating of the terminals or the lamp, and there is,a dark space around and in front of each electrode. This space and its appearance differ with different vapors used.

When the lamp was first shown last spring, it gave a harsh white light. This defect has been overcome by introducing traces of nitrogen with the mercury vapor, which has the effect of imparting a reddish tinge to the light that is very pleasant to the eye. The stability of the light under a constant pressure or voltage seems to be dependent upon maintaining a uniform vapor density or resistance to the current. This again is determined by the amount of heat developed and the rate at which the heat is radiated from the lamp. By suitable proportions of the metal vapor the density of heat radiation is maintained at a constant point, after the lamp has been once started, so that there is no variation in the illumination. During the early experiments with this type of lamp, in order to start the lamp glowing, it was found necessary to heat the lamp to a certain temperature before the current would begin to flow through it. This defect has been eliminated by a discovery that a small quantity of sulphur compound introduced in the lamp would enable the current to start very freely. In addition to this modification, a special starting device is used, by which the current is turned on the lamp, an initial current of high potential is forced through the lamp, when the low tension current will follow it, and the lamp will glow at its full intensity. As there is no solid conductor, such as the carbon filament used in the incandescent lamp, and no carbon electrodes, as in the case of the arc lamp, it would seem as though the Cooper-Hewitt lamp should show a remarkably long life. The efficiency is stated to be much higher than that of any lamp of the incandescent type, and the cost' of manufacture is small compared with the results obtained. What the effects of a prolonged test on this lamp are, has not been given out, and it is-impossible to state whether it will deteriorate through blackening or discoloring of the glass or decomposition of the light-giving vapors. The method of manufacturing this lamp, described briefly, is as follows: A tube of the proper shape is first cleansed with acids, alkali and water. It is then thoroughly rinsed with dilute hydrofluoric acid, and then washed with distilled water, and is sometimes given further cleansing in a bath of hot hydrogen gas. If mercury is to be used for the conducting vapor, a small quantity of it is placed in the tube ; and if sulphide of mercury is required for forming the starting material, a small amount of this substance is also added.

Pure sulphur has been used instead of sulphide of mercury, but it has been found more difficult to reach the desired results than with the mercury salt. The lamp is then connected to an exhaust pump, such as is used in the manufacture of ordinary incandescent lamps, aud while the exhaustion is taking place, artificial heat is applied to the tube in any convenient manner. After the air and other free gases have been exhausted, an electric current of high potential is applied to the terminals of the lamp. The operation of the exhaust pump is continued, and as it approaches completion, a voltage equal to that under which the lamp is to be operated, is applied to the terminals. When the proper stage has been reached, the lamp becomes intensely illuminous owing to the passage of this current, and after some further adjustment the lamp is sealed off the pump, and is ready for use after the connect, ors and supporting devices have been added. It is impossible to state at this early stage what position the Cooper-Hewitt lamp will occupy in the field of electric lighting. It seems to have advantages over the Nernst lamp in simplicity of construction and the ability to manufacture it for comparatively small candle-power. It does not seem, however, as though it is as practicable of such minute subdivision as the ordinary incandescent lamp, and it is probable that it will be used chiefly for general illumination, thus occupying an* intermediate position between the arc and incandescent systems.

At the present time it seems as though a distinct advance has been made in electric lighting, and future developments will be watched with great interest,not only by the general public, but by the manufacturers of electric-lighting apparatus and the central station managers.

Experiment has shown that an electric arc can be employed under water for fusing metal. The intense heat turns the water surrounding the arc into steam, thus forming an insulating cushion of vapor. It has been suggested that with proper apparatus the electric arc could be employed by divers for quickly cutting through large chain cables or iron plates under water.