lamp, though not yet adapted to all the purposes served by the Edison lamp, on account of its peculiar color, produces eight times as much light with the same amount of power. It is also practically indestructible, there being no filament to burn out; and it requires no special wiring. By means of this invention electricity, instead of being the most costly means of illumination becomes the cheapest-cheaper even than kerosene. No further explanation than this is necessary to show the enormous importance of this invention."

As just stated, the defect of the Edison incandescent lamp is its cost, due to its utilizing only a small fraction of the power used in producing the incandescence, and, of much less importance, the relatively short life of the filament itself. Only about three per cent. of the actual power is utilized by the light, the remaining ninety-seven per cent. being absolutely wasted; and it was this enormous waste of energy that first attracted the attention of Mr. Hewitt, and led him to direct his energies to finding a substitute that would be more economical. A large part of the waste in the Edison bulb is known to be due to the conversion of the energy into useless heat, instead of light, as shown by the heated glass. Mr. Hewitt attempted to produce a light that would use up the power in light alone-to produce a cool light, in short.

Instead of directing his efforts to the solids, Mr. Hewitt turned his attention to gascous bodies, believing that an incandescent gas would prove the more nearly ideal substance for a cool light. The field of the passage of electricity through gases was by no means a

virgin one, but was nevertheless relatively unexplored: and Mr. Hewitt was, therefore, for the most part obliged to depend upon his own researches and experiments. In these experiments hundreds of gases were examined, some of them giving encouraging results, but most of them presenting insurmountable difficulties. Finally mercury vapor was tried, with the result that the light just referred to was produced.

The possibilities of mercury-vapor gas had long been vaguely suspected-suspected, in fact, since the early days of electrical investigation, two centuries before. The English philosopher, Francis Hauksbee, as early as 1705 had shown that light could be produced by passing air through mercury in an exhausted receiver. He had discovered that when a blast of air was driven up against the sides of the glass receiver, it appeared "all round like a body of fire, consisting of an abundance of glowing globules," and continuing until the receiver was about half full of air. Hauksbee called this his "mercurial fountain," and although he was unable to account for the production of this peculiar light, which he remarked "resembled lightning," he attributed it to the action of electricity.

Between Hauksbee's "mercurial fountain" and Hewitt's mercury-vapor light, however, there is a wide gap, and, as it happened, this gap is practically unbridged by intermediate experiments, for Mr. Hewitt had never chanced to hear anything of Hauksbee's early experiments, or of any of the tentative ones of later scientists. But this, on the whole, may have been rather advantageous than otherwise, as, being ignorant,

he was perhaps in a more receptive state of mind than if hampered by false or prejudicial conceptions. Be this as it may, he began experimenting with mercury confined in a glass tube from which the air had been exhausted, the mercury being vaporized either by heating, or by a current of electricity. No results of any importance came of his numerous experiments for a time, but at last he made the all-important discovery that once the high resistance of the cold mercury was overcome, a comparatively weak current would then be conducted, producing a brilliant light from the glow of the mercury vapor. Here, then, was the secret of the use of mercury vapor for lighting-a powerful current of electricity for a fraction of a second passed through the vapor to overcome the initial resistance, and then the passage of an ordinary current to produce the light. In practice this apparent difficulty in overcoming the initial resistance with a strong current is easily overcome by the use of a "boosting coil," which supplies the strong current for an instant, and is then shut off automatically, the ordinary current continuing for producing the light. The mechanism is hardly more complex than that of the ordinary incandescent light, but the current of ordinary strength produces an illumination about eight times as intense as the ordinary incandescent bulb of equal candle-power.

The form of lamp used is that of a long, horizontal tube suspended overhead in the room, a brilliant light being diffused, which, lacking the red rays of ordinary lights, gives a bluish-green tone to objects, and a particularly ghastly and unpleasant appearance to faces and

hands, as referred to a moment ago. In many ways this feature of the light is really a peculiarity rather than a defect, and for practical purposes in work requiring continued eye-strain the absence of the red rays is frequently advantageous. In such close work as that of pen-drawing, for example, some artists find it advantageous to use globes filled with water tinted a faint green color, placed between the lamps and their paper, the effect produced being somewhat the same as that of the mercury-vapor light. For such work the absence of the red rays of the Hewitt light would not be considered a defect; and in workshops and offices where Mr. Hewitt's lamps are used the workmen have become enthusiastic over them.

On the other hand, the fact that the color-values of objects are so completely changed makes this light objectionable for ordinary use; so much so, in fact, that the inventor was led to take up the problem of introducing red rays in some manner so as to produce a pure white light. He has partly accomplished this by means of pink cloth colored with rhodium thrown around the glass; but this causes a distinct loss of brilliancy.

The most natural method of introducing the red rays, it would seem, would be to use globes of red glass; but a moment's reflection will show that this would not solve the difficulty. Red glass does not change light waves, but simply suppresses all but the red rays; and since there are no red rays in the mercuryvapor light the result of the red globe would be to suppress all the light. Obviously, therefore, this apparently simple method does not solve the difficulty; but those

familiar with Mr. Hewitt's work will not be surprised any day to hear that he has finally overcome all obstacles, and produced a perfectly white light. In the meantime the relatively expensive arc light and the incandescent bulb with its filament of carbon or metal hold unchallenged supremacy in the commercial field.

VOL. VI.-16