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air rushing in. At once they sealed the bottles shut again in the flickering flame of an alcohol lamp. They did the same stunt in the yard of the observatory with another ten bottles, then hurried back to the little laboratory to crawl under the stairs to put the bottles in the incubating oven.

A few days later Pasteur might have been seen squatting before his oven, handling his rows of flasks lovingly, laughing his triumph with one of those extremely rare laughs of his—he only laughed when he found out he was right. He put down tiny scrawls in his notebook, and then crawled out of his cubby-hole to tell his assistants: "Nine out of ten of the bottles we opened in the cellar of the Observatory are perfectly clear—not a single germ got into them. All the bottles we opened in the yard are cloudy—swarming with living creatures. It's the air that sucks them into the yeast soup—it’s the dust of the air they come in with!"

He gathered up the rest of the bottles and hurried to the train—it was the time of the summer vacation when other professors were resting—and he went to his old home in the Jura mountains and climbed the hill of Poupet and opened twenty bottles there. He went to Switzerland and perilously let the air hiss into twenty flasks on the slopes of Mont Blanc; and found, as he had hoped, that the higher he went, the fewer were the flasks of yeast soup that became cloudy with swarms of microbes. "That is as it ought to be," he cried, "the higher and clearer the air, the less dust—and the fewer the microbes that always stick to particles of the dust." He came back proudly to Paris and told the Academy—with proofs that would astonish everybody!—that it was now sure that air alone could never cause living things to rise in yeast soup. "Here are germs, right beside them there are none, a little further on there are different ones . . . and here where the air is perfectly calm there are none at all," he cried. Then once more he set a new stage for possible magnificent exploits: "I would have liked to have gone up in a balloon to open my bottles still higher up!" But he didn't go up in that