THE PROGRESS OF SCIENCE. 



x 93 



order that van't Hoff mighl lecture if 

 he wished, he was appointed to a pro- 

 fessorship at the University of Berlin, 

 also without specified duties. The re- 

 markable feature of this arrangement 

 is that it was primarily as a member 

 of the Academy that van't Hoff went 

 to Berlin. Xo such honor has been 

 paid a man in Germany since the time 

 of Frederick the Great. An outline 

 of van't HofT's career will prove that 

 the tribute was deserved. 



When only twenty-two years old 

 van't Hoff showed that certain unex- 

 plained 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 im- 

 portance 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 prop- 

 erty. The branch of chemistry known 

 as stereochemistry is the outgrowth 

 of the paper published by van't Hoff 

 in 1874 and of the independent state- 

 ment 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 chem- 

 istry 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 quantita- 

 tive 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 equilib- 

 rium 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 tem- 

 perature. 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 de- 

 clined; but the ideal position offered 

 by the Prussian Academy in the fol- 

 lowing year was accepted and van't 

 Hoff left Amsterdam in 1896 for Ber- 

 lin. 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 mathe- 

 matics 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 applica- 

 tion of mathematics is possible in this 

 field and there is the fascinating pros- 

 pect of applications to the Stassfurt 

 industry and to geology. 



