had no conception. In the opening years of the following century Francis Hauksbee obtained somewhat similar results with glass globes and tubes, and made several important discoveries as to the properties of electricity that stimulated an interest in the subject among the philosophers of the time. Gray in England, and Dufay in France, who became enthusiastic workers in the field, soon established important facts regarding conduction and insulation, and by the middle of the eighteenth century the production of an electric spark had become a commonplace demonstration.

But until this time it had not been demonstrated that this electric spark was actual fire, although there was no disputing the fact that it produced light. In 1744, however, this point was settled definitely by the German, Christian Friedrich Ludolff, who projected a spark from a rubbed glass rod upon the surface of a bowl of ether, causing the liquid to burst into flame. A few years later Benjamin Franklin demonstrated with his kite and key that lightning is a manifestation of electricity.

But neither the galvanic cell nor the dynamo had been invented at that time, and there was no possibility of producing anything like a sustained artificial light with the static electrical machines then in use. It was not until the classic discovery of Galvani and the resulting invention of the voltaic, or galvanic, cell shortly after, that the electric light, in the sense of a sustained light, became possible. And even then, as we shall see in a moment, such a light was too expensive to be of any use commercially.

DAVY AND THE FIRST ELECTRIC LIGHT

As soon as Volta's great invention was made known a new wave of enthusiasm in the field of electricity swept over the world, for the constant and relatively tractable current of the galvanic battery suggested possibilities not conceivable with the older friction machines. Batteries containing large numbers of cells were devised; one having two thousand such elements being constructed for Sir Humphry Davy at the Royal Institution, of London. By bringing two points of carbon, representing the two poles of the battery, close together, Davy caused a jet of flame to play between them— not a momentary spark, but a continuous light—a true voltaic arc, like that seen in the modern street-light to-day.

"When pieces of charcoal about an inch long and one-sixth of an inch in diameter were brought near each other (within the thirtieth or fortieth of an inch),' wrote Davy in describing this experiment, "a bright spark was produced, and more than half the volume of charcoal became ignited to whiteness; and, by withdrawing the points from each other, a constant discharge took place through the heated air, in a space equal to at least four inches, producing a most brilliant ascending arch of light, broad and conical in form in the middle. When any substance was introduced into this arch, it instantly became ignited; platina melted in it as readily as wax in a common candle; quartz, the sapphire, magnesia, lime, all entered into fusion; fragments of dia

mond and points of charcoal and plumbago seemed to evaporate in it, even when the connection was made in the receiver of an air-pump; but there was no evidence of their having previously undergone fusion. When the communication between the points positively and negatively electrified was made in the air rarefied in the receiver of the air-pump, the distance at which the discharge took place increased as the exhaustion was made; and when the atmosphere in the vessel supported only one-fourth of an inch of mercury in the barometrical gauge, the sparks passed through a space of nearly half an inch; and, by withdrawing the points from each other, the discharge was made through six or seven inches, producing a most brilliant coruscation of purple light; the charcoal became intensely ignited, and some platina wire attached to it fused with brilliant scintillations and fell in large globules upon the plate of the pump. All the phenomena of chemical decomposition were produced with intense rapidity by this combination."

It will be seen from this that as far as the actual lighting-part of Davy's apparatus was concerned, it was completely successful. But the source of the current -the most essential part of the apparatus-was such that even the wealthy could hardly afford to indulge in it as a luxury. The initial cost of two thousand cells was only a small item of expense compared with the cost of maintaining them in working order, and paying skilled operators to care for them. So that for the moment no practical results came from this demonstration, conclusive though it was, and the introduction of a com

mercial electric light was of necessity deferred until a cheaper method of generating electricity should be discovered.

This discovery was not made for another generation, but then, as seems entirely fitting, it was made by Davy's successor and former assistant at the Royal Institution, Sir Michael Faraday. His discovery of electromagnetic induction in 1831 for the first time made possible the electric dynamo, although still another generation passed before this invention took practical form. In the meantime, however, the magneto-electric machine of Nollet was used for generating an electric current for illuminating purposes as early as 1863; and when finally the dynamo-electric machine was produced by Gramme in 1870, engineers and inventors had at their disposal everything necessary for producing a practical electric illuminant.

It must not be supposed, however, that inventors stood by patiently with folded hands waiting for the coming of a machine that would furnish them with an adequate current without attempting to produce electric lamps. On the contrary, they were constantly wrestling with the problem, in some instances being fairly successful, even before the invention of the magneto-electric machine. Great advances had been made in batteries and cell construction over the primitive cells of the time of Davy, and for exhibition purposes, and even for lighting factories and large buildings, fairly good electric lights had been used before 1863.

The first practical application of electric lighting seems to have been made in France in 1849. During

the production of the opera "The Prophet" the sun was to appear, and for this purpose an electric arc light was used. The success of this effort-an artificial sun being produced that seemed almost as dazzling to the astonished audience as Old Sol himself-stimulated further efforts in the same direction. The previous year W. E. Staite in England made experiments along similar lines in the large hall of the hotel of Sunderland. He generated a light "resembling the sun, or the light of day, and making candles appear as obscure as they do by daylight," according to the Times of the following morning. The electric light was therefore proved to be a practical illuminator, although it was not until the introduction of the Gramme dynamo-electric machine that its great economic utility was demonstrated.

THE JABLOCHKOFF CANDLE

In Sir Humphry Davy's experiments with his arc light he was led to believe that the light between the two points of carbon would be produced even in an absolute vacuum, if it were possible to create one. Several scientists at the time disputed this contention, and M. Masson, Professor of Physics in the École Centrale des Arts et Manufactures in Paris was particularly active in combatting the idea, maintaining that the arc had the same cause as the electric spark-the transport by electricity of the incandescent particles of the electrodes through the atmosphere. It was certain, at any rate, that no light was produced when the opposing carbons were brought into contact with each