For all the long hot weeks of the next summer Dora worked faithfully every day on the hotel lamps. She had to be at her work at eight o'clock every morning, and she seldom finished before two in the afternoon. But every week her uncle paid her three dollars and a half, and by the end of the season she had forty-two dollars carefully put away. When the hotel closed, her uncle made her a present of eight dollars, so that when she started for school in the fall she rejoiced in the thought of fifty dollars put away in the savings bank as a college fund.
She was happy, too, in the prospect of making as much money the next summer. For the Wizard, Uncle John told her, had not the secret yet. He had succeeded in making a platinum wire, encased in a glass globe, give a light equal to that of twenty-five candles without melting. But he needed to exhaust all the air from the glass globe, and still one one-hundred-thousandth of the original volume remained.
"But that's not sufficient," commented Uncle John. "I know enough about the matter to be sure that so much air as that would prevent the platinum from giving out the light it ought to give. Still, within a short time, Dora, I expect even the Portland papers will describe Mr. Edison's success with the electric light."
Uncle John's prediction was fulfilled. By the first of October the vacuum was so nearly perfect that only one-millionth part of the original air was left in the glass bulb. By the last of that same month, moreover, the whole secret was practically in Edison's grasp. He had stopped experimenting with platinum for a burner and had gone back to carbon, on which he had pinned his faith at first.
But this time he used the carbon only as a coating for a piece of cotton thread that he had bent into a loop and sealed up in the almost perfect vacuum of glass. When this lamp was connected with the battery, it flashed forth with the brightness that the inventor had so long waited to see. But how long would it burn? There was no sleep for Edison till that question was answered; and it was not answered for forty hours—nearly two days of growing delight and diminishing anxiety.
Such a discovery meant the end of all fruitless experimenting. The secret of the incandescent light was revealed; and the newspapers all over the country—the Daily Eastern Argus of Portland among them—spread the knowledge of the great event in science and prophesied the speedy conquest of kerosene and gas. Late in November Uncle John sent Dora a copy of the Scientific American which gave the authoritative account of what had been accomplished.
Edison in his Library.
"But," wrote Uncle John in the letter accompanying the paper, "now the real work has only begun. The Wizard knows that some carbonized material is what he needs, but he is sure that carbonized cotton thread is not the best thing. Now he is carbonizing everything he can lay his hands on—straw, tissue paper, soft paper, all kinds of cardboard, all kinds of threads, fish line, threads rubbed with tarred lampblack, fine threads plaited together in strands, cotton soaked in boiling tar, lamp wick, twine, tar and lampblack mixed, and many other materials that I can't remember. Why," finished Uncle John, "so far he has examined no fewer than six thousand different species of vegetable growth alone. Somebody said something to him the other day about his wonderful genius. 'Well,' modestly answered the great man, mopping his forehead with his handkerchief, 'genius, I think, is one per cent inspiration and ninety-nine per cent perspiration.'"
In December there came into Dora's life the most happy and exciting experience of her childhood. The letter from Uncle John in November had ended with this paragraph: