How the Theory of Solutions Arose

by George Wald

Biological Laboratories, Harvard University, Cambridge, MA 02138

Journal of Chemical Education 23, 8 (August 1986)

Editor's Note: Nobel laureate George Wald is Professor Emeritus of Biology at Harvard University and spent many summers in research at the Marine Biological Laboratories, Woods Hole, Massachusetts. Portions of this essay originally appeared in a somewhat different form in the September 17, 1982 edition of Science.

When W. J. V. Osterhout (1871-1964) was a member emeritus of the Rockefeller Institute, he told this story to me during one of my visits to him and his wife and co-worker Marion (Ikky) Irwin, in their apartment at the Marine Biological Laboratories in Woods Hole. I begged him to write it out, for I thought it too interesting and important to be lost, but he never did. I shall tell it first as he told it to me, for I think he had it from the source, as we shall see. Then I will authenticate it, as I now can do. I wish to dedicate this essay to his memory.

One day in Amsterdam, Jacobus Henricus van't Hoff (1852-1911), the "father of physical chemistry", was walking down the street from his laboratory when he encountered his fellow professor, the botanist Hugo de Vries, out walking with his wife. Having met, they went on together, whereupon de Vries ventured, "The other day I had a letter from Pfeffer" [Wilhelm Pfeffer (1845-1920), the botanist who pioneered the use of semipermeable membranes to measure osmotic pressure]. When van't Hoff inquired in the desultory Dutch equivalent of "Oh, yeah? What's he up to?" de Vries replied, "He says he's measuring the effect of temperature on osmotic pressure." "What does he get?" asked van't Hoff. "Well," replied de Vries, "he writes that for each degree rise in temperature the osmotic pressure goes up by about 1/270."

That did it, for van't Hoff immediately recognized 270 to be an approximation of the absolute temperature, 273 K at O °C. By that night van't Hoff was well launched on the theory of ideal solutions, with its fundamental equation the exact equivalent of the ideal gas law, pv = RT, becoming in dilute solutions p / c = RT, in which p is now the osmotic pressure, c the concentration, R the universal gas constant, and T the absolute temperature.

So much from Osterhout. And now I shall authenticate the story. Having already become reconciled to its loss, I was happy to discover it mentioned in Ernst Cohen's monumental biography of van't Hoff.1 This in turn led me to van't Hoff's own telling of the story in a paper titled, "How the Theory of Solutions Arose", a special lecture to the German Chemical Society at its January 8, 1894, session, with Emil Fischer in the chair.2

Van't Hoff explains that he had come across a measurement by Eilhard Mitscherlich (1794-1863) that troubled him.3 Mitscherlich had tried to measure the strength of binding of water of hydration in salts by noting the lowering of the vapor pressure of water in their crystals. He put crystals of Glauber's salt (Na2SO4 . 10 H2O) into the vacuum space of a barometer and noted that the mercury sank 5.45 mm, whereas water would have made it sink 8.72 mm. He concluded that the difference, 3.27 mm, measures the affinity of Na2SO4 for its water of hydration. This would amount to a binding force of about 1/32 kg per 2.615 cm2 (about 12 g per cm2). Having quoted Mitscherlich to this effect, van't Hoff goes on to say, "This value, 1/200 atm, seems to me unprecedentedly low, since I have the impression that even the weakest chemical forces are very large, as seems to me also to be concluded from Helmholtz's Faraday lecture." Would not aqueous solutions offer a much simpler way to make such measurements than dealing with the state of water in crystals?

Van't Hoff continued. "Leaving the laboratory with this question on my lips, I encountered my colleague de Vries and his wife, who was just then busy with osmosis experiments and acquainted me with Pfeffer's measurements." Van't Hoff then goes into a discussion reconciling Mitscherlich's measurements with Pfeffer's, ending with the words:

So it occurred to me that with the semipermeable barrier all the reversible transformations that so materially ease the application of thermodynamics to gases, become equally available for solutions....
That was a ray of light, and led at once to the inescapable conclusion that the osmotic pressure of dilute solutions must vary with temperature entirely as does gas pressure, that is, in accord with Gay-Lussac's Law [i.e., p directly proportional to T].
There followed at once however a second relationship, which Pfeffer had already drawn close to: the osmotic pressure of dilute solutions is proportional also to concentration, i.e., alongside Gay-Lussac's Law, that of Boyle applies. Without doubt the famous mathematical expression pv = RT holds for both. With that in hand I could demonstrate my [further thermodynamic formulation], and had achieved my goal.

So the essential story is authentic. How about Osterhout's circumstantial details? Had he -- a good raconteur -- improvised them? No, I think that they too are authentic.

Here I have recourse to Lawrence Blink's biographical memoir on Osterhout, his former teacher.4 Osterhout taught at the University of California in Berkeley from 1896 until 1909 in the company of "sophisticated colleagues who remembered Pfeffer's laboratory". Berkeley president Benjamin Ide Wheeler, eager to build up the intellectual status of the still-young university, sponsored one-year fellowships for outstanding European scholars. Among these were de Vries, Arrhenius, and Ostwald. "There exists a photograph taken in 1905 showing de Vries beside an Oenothera plant in the botanic garden, flanked with the portly Arrhenius [Svante Arrhenius (1859-1927), who was the prototype of Sondelius in Sinclair Lewis' "Arrowsmith"] ... with [Jacques] Loeb smiling beside them, and Osterhout [in bowler hat] in the back row." This rare photograph is reproduced here (see figure). So Osterhout got to know de Vries and I think had the story, trimmings and all, from him.

Picture caption: Examining an Oenothera species in the Botanical Garden, University of California, Berkeley, 1904. Front row, left to right: E.J. Wickson, J. Loeb, E. Hilgard, H. de Vries, and S. Arrhenius. W. Osterhout is visible in the back row, just over Wickson's shoulder, to the left, in the bowler hat. (Photo courtesy of The Bancroft Library, University of California, Berkeley.)

Osterhout died at 92, de Vries at 87, Pfeffer at 75, but van't Hoff at 59. Is botany that much more preservative than physical chemistry? To be sure, van't Hoff drove himself hard and to the very end, but it was not physical chemistry that killed him. "Something seems to have altered my constitution," he wrote on August 1, 1906,1 and on March 1, 1911, he died of tuberculosis. Even an enforced stay in a sanatorium led to a paper, "Sanatoriums-Betrachtung", on heat and work relationships in active life and in the bedridden.5

1. Cohen, E. "Jacobus Henricus van't Hoff, sein Leben und Wirken"; Akad. Verlagsgesellschaft: Leipzig, 1912; pp 205-206.

2. Van't Hoff, J. H. Ber. Deutsch. Chem. Ges. 1894, 27, 6.

3. Mitscherlich, E. "Lehrbuch der Chemie," 1829 (4th ed., 1847); p 565

4. Blinks, L. R. Biogr. Mem. Natl. Acad. Sci. 1974, 44, 213.

5. Van't Hoff, J. H. "Biochemische Zeitschrift, Festband für H. J. Hamburger"; Springer: Berlin, 1908; p 260.