Spallanzanis Experiment

At about the same time that Redi carried out his experiment, other scientists began using a new tool—the microscope. Their observations with the microscope revealed that the world is teeming with tiny creatures. They discovered that microorganisms are simple in structure and amazingly numerous and widespread. Many investigators at the time thus concluded that microorganisms arise spontaneously from a "vital force" in the air.

In the 1700s, another Italian scientist, Lazzaro Spallanzani (1729-1799), designed an experiment to test the hypothesis of spontaneous generation of microorganisms, as shown in Figure 14-2. Spallanzani hypothesized that microorganisms formed not from air but from other microorganisms. He knew that microorganisms grew easily in food, such as broth made from boiled meat. Spallanzani reasoned that boiling broth in a flask would kill all the microorganisms in the broth, on the inside of the glass, and in the air in the flask. For his experimental group, Spallanzani boiled clear, fresh broth until the flasks filled with steam. While the broth was hot, he sealed the flasks by melting their glass necks. The control-group flasks of broth were left open. The broth in the sealed flasks remained clear and free of microorganisms, while the broth in the open flasks became cloudy because it was contaminated with microorganisms.

Spallanzani concluded that the boiled broth became contaminated only when microorganisms from the air entered the flask. Spallanzani's opponents, however, objected to his method and disagreed with his conclusions. They claimed that Spallanzani had heated the experimental flasks too long, destroying the "vital force" in the air inside them. Air lacking this "vital force," they claimed, could not generate life. Thus, those who believed in spontaneous generation of microorganisms kept the idea alive for another century.

Broth is boiled.

Broth remains clear.

Broth is boiled.

Broth remains clear.

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