Born about a decade before the French Revolution, Joseph Gay-Lussac was only 14 when his comfortable family life was disrupted by his father's arrest for suspected royalist sympathies. Although Gay-Lussac's tutor fled the country, the boy's education resumed at a private school in Paris, and he successfully competed for entrance to the new (cole Polytechnique. The hallmark of his scientific career was the analysis of gases, which he studied not only in the laboratory, using ingenious new techniques, but also on daring balloon flights that reached record altitudes.

Gay-Lussac's career began with a major revelation. In 1802, when he was just 24, he published a new fundamental law of physics that helps explain the behavior of gases. Although everyone knew that heat would make gases expand in volume, Gay-Lussac's careful experiments proved that different gases would all expand by the same amount with the same rise in temperature. Now known as...