The Francesco Redi Experiment
Francesco Redi was able to disprove the theory that maggots could be spontaneously generated from meat using a controlled experiment. Spontaneous generation, the theory that life forms can be generated from inanimate objects, had been around since at least the time of Aristotle. Francesco took eight jars, placed meat in all the jars, but covered four of the jars with muslin. Maggots developed in the open jars but did not develop in the muslin-covered jars. Today controlled experiments are commonly demanded by scientific journals and are sometimes legally required by regulatory bodies (especially for pharmaceuticals). The image below is taken from Esperienze intorno alla generazione degl' Insetti (p. 187) where Francesco Redi published a description of the experiment in 1668 (see sidebar for digital copies of book).
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Francesco Redi is often credited with developing the controlled experiment just as Galileo is often credited with introducing the modern experimental method. Both beliefs may be simplistic, however. Francesco Redi and Galileo Galileo demonstrated the effectiveness of their methods using very simple experiments, then explained their procedures and results in clear and compelling ways. These experiments were watershed events in the history of science. That is not to say that experimental scientists before Francesco Redi didn't realize the need to control variables or that the sequence of steps involved in Galileo's method hadn't been described by earlier natural philosophers (e.g. professors from the School of Padua). When Galileo was still a young boy, Giuseppe Moletti, the professor who he would eventually succeed at the University of Padua, decided to conduct some experiments on free fall by dropping weights in different media (see Timeline of Classical Mechanics). His test with free fall in water and air specified that the balls must be of the same substance, weight and figure in order to remove doubt. In the same book, when Moletti described dropping balls of wood and lead from a tower to demonstrate that free fall doesn't depend on weight (as Aristotle had said) he was careful to eliminate size as a nuisance variable by conducting the experiment with wooden balls of different sizes . Moletti's experiments certainly weren't as elegant as Francesco Redi's, but they indicate that he understood the need for controls .
Being careful to control for the known variables doesn't guarantee that you will get the correct results. That is because "you don't know what you don't know". What if there are variables that need to be controlled that you don't even know exist. This could explain why the famous Tower of Pisa experiment actually came up with incorrect results. Many consider the legend of the Tower of Pisa experiment to be a myth. The experiment did occur. It was conducted by Vincenzio Renieri, a Catholic monk (see Galileo's Battle for the Heaven's) and not Galileo as is commonly thought. Vincenzio was a friend of Galileo's. Like Moletti before him, Renieri, controlled for size when he dropped two balls of the same size (one of wood and one of lead.) He came up with the wrong results. There was almost 2 metres difference between the heavier and lighter balls when they hit the ground. Galileo described similar results in some of his works. These scientists could not have known that they needed to control for human physiology as well. Modern experiments with humans dropping balls of markedly different weights show that there is a tendency to grip the heavier ball more tightly and release it more slowly .
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