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Rh happily until seized with a sudden and violent fever. He expired August 21, 1814, in the sixty-second year of his age.

Baron Cuvier, the count's intimate and confidential friend, perpetual secretary of the French Institute, delivered the Éloge before his associates, in which he said: "It is an honor to France that a man who was held in such high esteem in the two most civilized continents, should choose France for a final sojourn. Here fuller celebrity is most surely awarded, regardless of favors of courts or the freaks of fortune." Here for ten years he had been honored by Frenchmen and by foreigners.

As a philosopher and physicist, Rumford ranks among the greatest of a period that was prolific in scientific research, discovery and invention. Before he began his investigations, the world was in almost absolute ignorance regarding the nature of heat. The metaphysical philosophy of the Greeks regarding the nature of heat, which had remained undisputed for two thousand years, was by Rumford overthrown and the phenomenon established on a mechanical basis. He disproved the theory of an "igneous fluid," or caloric, and conclusively proved that heat could not be a material body. He was the first to point out that energy and heat are mutually convertible, and that both are forms of motion. He was thus the founder of our modern science of thermodynamics.

While superintending the boring of brass cannons in the Arsenal at Munich, his attention was drawn to the great amount of heat acquired by the cannon and the high temperature of the brass chips. He investigated the source of this heat. A blunt borer was pressed with great force against the bottom of the bore hole in the cylindrical riser of a brass cannon. The cannon was rotated nearly a thousand times and the heat developed was sufficient to raise the whole cylinder, which weighed 113 pounds, to seventy degrees, while the amount of metal rubbed off by the borer was only 837 grains. He concluded that the supply of heat obtained from a given quantity of metal was inexhaustible, and hence heat could not be a material substance, but must be "motion."

He had a talent for inventing scientific instruments for making experiments. He thus invented the photometer, an instrument for measuring the relative intensity of different lights; the calorimeter, for measuring the quantity of heat; the thermoscope for indicating the difference of temperatures. He was an accomplished linguist, speaking and writing French, German, Italian and English with equal facility.

Rumford was buried at Auteuil. His monument bears the following inscription: