Mr. Edison found difficulties in his way. One was the apparent impossibility of illuminating by means of an incandescent solid, for even platinum will melt at a heat too low for use. But this apparent impossibility was overcome by the inventor’s genius. He, after many trials, found that if he raised the platinum to a white heat _in a vacuum_ he would practically obtain _a new metal_ which would sustain the required heat. [Illustration: Fig. 273.—Edison’s platinum lamp.] “In making an electric lamp without a regulator,” says Mr. Upton, “two things are essential,—great resistance in the wire, and a small radiating surface. Mr. Edison sought to combine these two essential conditions by using a considerable quantity of insulated platinum wire wound like thread on a spool.” This platinum, as shown in the accompanying cut (fig. 273), was suspended in a glass bulb _in vacuo_, the air contained in it being expelled by electricity, heating it, and suddenly cooling the platinum, and squeezing out the air by the process. But, after all, the great difficulty of the inventor was to insulate his wires so perfectly that they would not meet and become a conductor. For, to perfect his lamp, this non-conducting principle was a necessity, otherwise the current would flow across instead of going all along the wires. He had previously made many uses of carbon, which we know is infusible. He tried lampblack tar, but it contained air, and would not do. Thread answered his purpose, but was too fragile and uneven in texture. It suddenly occurred to him that paper—_charred paper_—cut into a thread-like form would satisfy all his conditions. The problem was solved—the lamp was a fact. But how can paper, so easily burned, answer? We will endeavour to explain. “A piece of charred paper, cut into horse-shoe shape, so delicate that it looked like a fine wire firmly clamped to the two ends of the conducting and discharging wires, so as to form part of the electric circuit, proved to be the long-sought combination.” We will now explain the construction of this little lamp, which is shown in the illustration (fig. 274) one-half of its actual size. The illuminating is equal to ten or twelve ordinary gas jets. [Illustration: Fig. 274.—Edison’s electric lamp.] The manner in which the paper is prepared is, like many other very important inventions, extremely simple, and, we may add, almost costless. Cardboard will furnish us with the loops, and these “horseshoes” are placed in layers in an iron box with tissue paper between each. The box is then hermetically sealed, and made red hot. The carbonized paper remains till all the air has been got rid of, and although it will burn freely to ashes in atmospheric air, _in the vacuum prepared for it_ it is never consumed. That is the plain fact—the secret of the Edison lamp.