Henri Victor Regnault (1810–1878) was one of the most famous French experimental scientists of the nineteenth century. After studying and carrying out research at the École Polytechnique and the École des Mines in Paris, he was elected to the Paris Académie des Sciences in 1840 and was appointed Professor of Experimental Physics at the Collège de France in 1841. His initial researches were in chemistry, but his careful experimental investigations of the law of the specific heat of solids that Pierre Louis Dulong (1785–1838) and Alexis Thérèse Petit (1791–1820) proposed in 1818 opened the door to his transition to physics and to his pioneering experimental researches on various thermodynamic properties of liquids and gases. I focus particularly on his investigations on the expansion, compressibility, vapor pressure, and speed of sound in gases. He also made important contributions to the new art of photography and to the ceramic industry as director of the Sèvres factory, at a time when his personal life was filled with tragedy. While his experimental work was acclaimed by his contemporaries, it has been largely neglected by scientists and historians today.