In 1812, Sir Humphry Davy highlighted for readers new to chemistry a three-step path to scientific knowledge. Observation amasses diverse facts; Analogy suggests ways to generalise about those facts; and any generalisations are then tested rigorously by Experiment. As an illustration, he proposed to examine the air bubbles that form on pond weed on sunny days.

Davy described a simple instance of observation, analogy and experiment. He remarked, by Analogy, that water weeds of all kinds produced the little gas bubbles he had noted by Observation in one sample. He then showed by Experiment that the gas consistently burns more brightly than air, before adding that such scientific method was a recent innovation.

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