order that van't Hoff might lecture if he wished, he was appointed to a professorship at the University of Berlin, also without specified duties. The remarkable feature of this arrangement is that it was primarily as a member of the Academy that van't Hoff went to Berlin. No such honor has been paid a man in Germany since the time of Frederick the Great. An outline of van't Hoff's career will prove that the tribute was deserved.
When only twenty-two years old van't Hoff showed that certain unexplained cases of isomerism would be accounted for if structure formulas were so written as to represent the arrangement of atoms in space and not merely relations in a plane. The importance of this new point of view lay in the fact that it enabled chemists to classify substances which rotate the plane of polarized light and to predict what substances will possess this property. The branch of chemistry known as stereochemistry is the outgrowth of the paper published by van't Hoff in 1874 and of the independent statement of the same idea by LeBel a few months later.
In 1878 van't Hoff was appointed professor of chemistry, mineralogy and geology at the new University of Amsterdam. From this time forward his work has been in physical chemistry rather than in organic chemistry. In the next six years he rediscovered the law of mass action; he worked out the generalized theory of reaction velocities; he showed that the quantitative relation between chemical affinity and heat effect has the same form as the relation between electrical energy and heat effect deduced by Helmholtz. In addition to this he established the theorem which bears his name, on the quantitative displacement of equilibrium with change of temperature.
In 1885 a new period begins. Some experiments by the botanist, Pfeffer, were the starting-point. Pfeffer had been studying the rise of sap in trees and had found that a high pressure is necessary to prevent the diffusion of water through a membrane of colloidal copper ferrocyanide into a solution of sugar in water. Van't Hoff showed that the results of Pfeffer could be predicted if it were assumed that a dissolved substance exercises an osmotic pressure equal to the pressure which it would exert if converted completely into a gas occupying the volume of the solution and having the same temperature. This assumption not only explained Pfeffer's results; but also those of Raoult on the vapor-pressures, boiling-points and freezing-points of solutions. When the osmotic pressure theory of solutions was supplemented by Arrhenius's theory of electrolytic dissociation, it needed only the energy and enthusiasm of Ostwald to raise physical chemistry in the short space of twenty years to the position which it now holds.
In 1894 van't Hoff was offered the chair of physics at Berlin, made vacant by the death of Kundt. This was declined; but the ideal position offered by the Prussian Academy in the following year was accepted and van't Hoff left Amsterdam in 1896 for Berlin. Since that time he has worked systematically at a problem which had interested him off and on for many years previously. What this problem is can be learned from van*t Hoff's own outline of his plans in an address before the Prussian Academy on July 2, 1896.
"The line along which I shall work is clear; the application of mathematics to chemistry remains my chief aim, and each opportunity to promote this in my new surroundings will be welcome. For the present, therefore, I shall devote myself to that portion of physical chemistry which deals with the so-called inversion phenomena, with the formation of double salts, and with double decomposition. The application of mathematics is possible in this field and there is the fascinating prospect of applications to the Stassfurt industry and to geology.
