WILHELMY, LUDWIG FERDINAND

(b. Stargard, Pomerania [now Poland], 25 December 1812; d. Berlin, Germany, 18 February 1864)

physics, chemistry.

After completing his early schooling, Wilhelmy left Pomerania to study pharmacy in Berlin. He subsequently purchased an apothecary shop in his native state and joined his father in business. His desire for pure scientific research led him to sell the shop in 1843, however, and to study chemistry and physics at Berlin, Giessen, and Heidelberg. In 1846 Wilhelmy received the doctorate from Heidelberg on the basis of a dissertation on heat as a measure of cohesion. After traveling chiefly in Italy and Paris, where he studied with Regnault, he returned to Heidelberg and became a Privatdozent in 1849. He remained at the university for only five years; he then returned to private life in Berlin after a six-month stay in Munich, keeping busy with philosophical, mathematical, and physical studies. Wilhelmy never married, preferring to devote his entire attention to expanding his knowledge in all areas of learning. A skilled businessman, he had a warm heart and was always willing to assist a friend. Although shy with strangers, in his small circle of colleagues he was cheerful and witty, defending his peculiar views on many subjects with surprising liveliness.

As a student in Berlin, Wilhelmy joined Magnus in forming a physics colloquium that became the Physical Society in 1845. Among the members of this small circle of young investigators were Paul du Bois-Reymond, Clausius, Helmholtz, and Werner Siemens. Upon his return to Berlin ten years later, Wilhelmy found few of the original group remaining; and he consequently took the lead in directing the younger members. As leader of the Physical Society, he converted part of his Berlin home, as well as his summer villa in Heidelberg, into physics laboratories in 1860. His studies on capillary action, unfinished at his death, were carried out at his home laboratory.

Wilhelmy is best known as the first person to measure the velocity of a homogeneous chemical reaction. In 1850 he published a paper on the law of the action of acids on cane sugar, which went virtually unnoticed until Wilhelm Ostwald called attention to it thirty-four years later. Wilhelmy’s procedure involved following the reaction with a polarimeter (widely used at the time of his investigations), which did not disturb the conditions of the reacting system. In the presence of a large amount (considered constant) of water, he found that the amount of sugar changed in any instant was proportional to the amount present, the acid being unchanged. In mathematical terms Wilhelmy presented the familiar law

–dZ/dt = MZS,

where Z is the concentration of sugar, t is time, S is the acid concentration (presumed unchanging throughout the reaction), and M is a constant today called the reaction velocity constant. He also investigated the temperature dependence of the reaction and assumed that it followed the same exponential law as concentration.

Wilhelmy’s earlier physical studies, dealing with heat and utilizing differential equations, prepared the way for his key 1850 paper. In his dissertation (1846) he used Regnault’s coefficients of expansion to calculate the force of cohesion and concluded that molecules are acted upon by two forces, heat and cohesion, the former tending to annihilate the latter. His conclusions are reminiscent of Lavoisier’s ideas about calorique. Wilhelmy added to these conclusions the concept of the numerical equivalence of heat and energy, following J. R. Mayer’s 1842 study of this equivalence. In a book published in 1851 Wilhelmy attempted to derive several general relationships among physical properties of compounds. He suggested, for example, that isomeric compounds of equal specific gravity and equal boiling points have equal coefficients of expansion. If the boiling points are different, the coefficients of expansion are inversely proportional to them. The book was meant to be an introduction to a work that would provide a complete understanding of the essence of natural forces, but this ambitious project was never completed.

BIBLIOGRAPHY

I. Original Works. Wilhelmy’s important paper on the rate of inversion of sugar, “Ueber das Gesetz, nach welchem die Einwirkung der Säuren auf den Rohrzucker stattfindet,” in Annalen der Physik und Chemie,81 (1850), 413–433, 499–526, was reprinted as Ostwald’s Klassiker der exacten Wissenschaften, no. 29 (Leipzig, 1891), which contains a complete list of Wilhelmy’s ten publications. Portions of the paper were translated into English in Henry M. Leicester and H. S. Klickstein, A Source Book in Chemistry 1400–1900 (New York, 1952), 396–400. In addition to his book on heat, Versuch einer mathematisch-physikalischen Wärmetheorie (Heidelberg, 1851), he published one other book, Zur physikalischen Begründung der Physiologie und Psychologie (Heidelberg, 1852).

II. Secondary Literature. A short biographical sketch of Wilhelmy by Georg Quincke is in Ostwald’s Klassiker (see above), 45–47. His work is discussed in Eduard Farber, “Early Studies Concerning Time in Chemical Reactions,” in Chymia,7 (1961), 135–148.

Sheldon J. Kopperl