March i6, 191 i] 



NATURE 



85 



famous "Principle of Mobile Equilibrium " : — '"Tout 

 equilibre entre deux etats differents de la matiere 

 (systemes) se deplace par un abaissement de la tem- 

 perature du cote de celui des deux systemes, dont la 

 formation developpe de la chaleur." 



Finally, in the chapter on affinity, perhaps the most 

 remarkable and original part of the whole book, 

 van 't Hoff shows for the first time how the chemical 

 forces operative in reaction can be measured and com- 

 pared. 



But van 't Hoff's greatest triumph was still to 

 come, though we may regard the application of 

 thermodynamics to chemical equilibrium to be found 

 in the "Etudes" as preparing the way for it. In 

 1886 van 't Hoff's famous paper on "The Laws of 

 Chemical Equilibrium in the Dilute, Gaseous Qr Dis- 

 solved State of Matter " was published in the Trans- 

 actions of the Swedish Academy of Sciences. This 

 was quickly followed by two other fundamental 

 papers, one on "A General Property of Dilute 

 Matter," and one on "Electrical Conditions of 

 Chemical Equilibrium." In 1887 there appeared in 

 the first volume of the " Zeitschrift fiir physikalische 

 Chemie " an abstract of the above, bearing the title 

 " Die Rolle des osmotischen Draches in der Analogic 

 zwischen Losungen und Gasen." 



It would be impossible to exaggerate the funda- 

 mental importance of these researches. By developing 

 the idea of osmotic pressure on the basis of the ex- 

 perimental work of Traube and Pfeffer and by an 

 application of the laws of thermodynamics, van 't 

 Hoff was enabled to put the whole subject of physical 

 and chemical equilibrium in dilute solutions on a sure 

 and simple basis. The extraordinary analogy which 

 he discovered, and showed also to be thermo- 

 dynamically necessary, between the laws connecting 

 the osmotic pressure, molecular concentration, and 

 temperature of a dilute solution and the corresponding 

 simple gas laws, played a profound part in this 

 development. 



Although the laws of equilibrium in solutions had 

 been given by Gibbs previously in a generalised 

 thermodvnamlcal form, and had been applied by 

 Helmholtz to cases where the partial vapour-pressures 

 as functions of the concentration were empirically 

 known, van 't Hoff was the first, by means of the 

 concept of osmotic pressure and the simple laws relat- 

 ing to it, to create a practically useful thermo- 

 dynamical theory of dilute solutions. In doing this 

 he demonstrated the insufficiency of Guldberg and 

 Waage's laws in its original form, and paved the way 

 for the electrolytic dissociation theory of Arrhenius 

 and its development and application by Ostwald and 

 Nernst. In fact, the simple interpretation which the 

 theory of electrolytic dissociation gives to the co- 

 efficient i of van 't Hoff's thermodj'namical equations 

 remains to-day, as it was twenty years ago, one of 

 the chief foundations of this theory. 



The theory of dilute solutions, as developed by 

 van 't Hoff, rendered inestimable service to the 

 general development of chemical science in the sure 

 tliermodynamical basis which it gave to the previously 

 empirical methods for determining the molecular 

 weights of dissolved substances. It would be difficult 

 indeed at the present day to discover any branch of 

 chemical, physiological, or biological science which 

 does not owe something to the fundamental advances 

 made by van 't Hoff in the theory of physical and 

 chemical equilibrium in dilute solutions. 



The last great period of van 't Hoff's scientific 

 activity was mainly devoted to a study of hetero- 

 geneous equilibrium, especially as regards the condi- 

 tions determining the formation and decomposition 

 of double salts, and the crystallisation of complex 



NO. 2159, VOL. 86] 



mixtures. The first fruits of these researches ap- 

 peared in collected form in his " Vorlesungen iiber 

 Bildung und Spaltung von Doppelsalzen " (Berlin, 

 1897). Here for the first time were set forth in lucid 

 and masterly manner all the conditions of tempera- 

 ture and concentration which determine the crystallisa- 

 tion of double and single salts, with many and varied 

 applications to special cases, culminating in the un- 

 ravelling of the complicated phenomena presented by 

 the double racemates of Scacchi and Wyrouboft. 

 Another striking case was the working out of the 

 conditions of formation of Schonite. The book also 

 contains an account of the beautiful experimental 

 methods worked out by van 't Hoff for the deter- 

 mination of transition-points. 



The researches summarised in this book may be 

 regarded as forming a fitting prelude to the great 

 work which van 't Hoff undertook during his resi- 

 dence in Berlin, namely, the investigation of the 

 formation of oceanic salt deposits, with special refer- 

 ence to those occurring at Stassfurth. This great 

 geological problem was undertaken in collaboration 

 with Aleyerhoffer, and with the help of a number of 

 research students. It reduced itself practically to the 

 determination of the heterogeneous equilibria occur- 

 ring in solutions containing the chlorides, sulphates, 

 and borates of sodium, potassium, calcium, and 

 magnesium. Proceeding step b}' step from the simple 

 to the more complex solutions, and employing all the 

 methods which he had previously worked out for 

 determining equilibria and transition-points, van 't 

 Hoff slowly but surely succeeded in unravelling the 

 gigantic problem which he had set himself to solve. 

 It would be quite impossible here to convey even a 

 faint idea of the complexity of the task, and of the 

 genius which enabled van 't Hoff to attack it so 

 successfully. Probably no part of his life-work is 

 less generally known than the wonderful series of 

 researches which issued from his laboratory at Char- 

 lottenburg during the 3-ears 1896-1909, and yet it 

 may be safely said that these researches constitute an 

 absolutely fundamental advance not only in physical 

 and inorganic chemistry, but also in scientific mineral 

 synthesis and rational experitnental geology. They 

 will for ever remain the classical model for the 

 application of the methods and principles of hetero- 

 geneous chemical equilibrium to the science of experi- 

 mental mineralogy. Van 't Hoff published a collected 

 account of these researches in two small volumes with 

 the modest title, " Zur Bildung der ozeanischen 

 Salzablagerungen " (Vieweg, Brunswick, 1905 and 

 1909.) This magnificent work on the Stassfurth salt 

 deposits was the last great problem to which van 't 

 Hoff devoted his attention. It forms a fitting close 

 to a life of strenuous work and extraordinary scientific 

 fertility. 



In spite of the continuous labour involved in his 

 daily work in the laboratory and in the publication 

 of his researches, van 't Hoff found time, during his 

 residence in Berlin, to publish his "Vorlesungen iiber 

 theoretische und physikalische Chemie " (Vieweg. 

 Braunschweig). This will long remain a standard 

 work on theoretical chemistry. It is characterised In- 

 great originalitv of treatment, remarkable breadth of 

 outlook, and that close and intimate relationship of 

 fact and theory which was always one of van 't Hoff's 

 salient characteristics. 



Of other works may be mentioned a short sum- 

 mary of the "Theory of Solutions," published in 

 Ahrens' " Sammlung chemischer Vortriige " (1900). 

 and the delightful "Acht Vortriige iiber physikalische 

 Chemie " ( Vieweg, IQ02), an embodiment of the 

 lectures which van 't Hoff was invited to give at the 

 L^niversity of Chicago in 1901 on the occasion of 



