IN the small town of Heilbronn, in Würtemberg, stands a monument erected to the memory of. the physician, Robert Mayer. It was unveiled in 1892, just fifty years after the publication of Robert Mayer's first essay on the conservation of energy. His career as a scientific discoverer is marked by many pathetic incidents. After the study of medicine he was made sanitary officer on a Dutch vessel, bent for the East Indies. During the long ocean voyage on the slow sailing vessel he was left much to himself. He gave his leisure hours to the contemplation of scientific subjects. He had occasion to observe that, in tropical countries, blood taken from the veins of patients looks almost like arterial blood. He concluded that in the tropics less oxidation is necessary than in a cold climate, in order to maintain a uniform bodily temperature. There must be a quantitative relation between the amount of heat generated and the temperature in which we live. In cold, northern climates "more heat must be developed for the maintenance of uniform bodily temperature. During the 219 days between February and September, 1840, spent on the water, Mayer dwelt in close intellectual communion with nature, and she gradually revealed to him one of her most precious secrets. Upon his return to Heilbronn he kept on thinking. A moving body is brought to rest by friction; heat appears. Has the motion disappeared into nothing? Has heat sprung out of nothing? If not, then there must be an equivalence between the heat generated and the motion destroyed. Causa æquat affectum, "Cause is equal to effect" became his favorite axiom. At first he thought that kinetic energy varied as the velocity. Later he recognized his error and perceived its variance with the square of the velocity.
On June 16, 1841, he sent an essay embodying his new ideas for