# Popular Science Monthly/Volume 79/July 1911/Jacobus Henricus Van't Hoff

Layout 4
* JACOBUS HENRICUS VAN'T HOFF
By Professor HARRY C. JONES
JOHNS HOPKINS UNIVERSITY
THE death of Van't Hoff removes one of the leading men of science not only of this age, but of all time.
Born in Rotterdam in 1852, the son of a physician, he received his early training in the Bealschule in Rotterdam and in the Polytechnikum in Delft. At twenty he had completed his work in the University of Leiden. He then studied under Kekulé in Bonn and Würtz in Paris, and obtained the Doctor's degree at Utrecht, at the age of twenty-two.
Van't Hoff, in 1876, was appointed privatdozent in physics in the veterinary college in Utrecht. In 1877 he was called to Amsterdam as lecturer in chemistry, and was appointed professor of chemistry in 1878, a position which he held until 1896, when he accepted a call to a chair created for him at Berlin. He held this chair and was also a member of the Berlin Academy of Sciences until his death on March 1, 1911.
Van't Hoff did three great things. His early work was with Mulder in Utrecht, Kekulé in Bonn and Würtz in Paris, and therefore, in organic chemistry. He raised and answered the question, what is the arrangement in three dimensions in space of the atoms in the simpler compounds of carbon? Henri had shown that the four hydrogen atoms in marsh gas (CH$_4$) all bear the same relation to the molecule. The geometrical configuration of this molecule follows of necessity from this fact. The only geometrical form in three dimensions in space fulfilling the condition of a central object surrounded symmetrically by four things of the same kind, is the regular tetrahedron. Thus arose the theory of the "tetrahedral carbon atom."
Some compounds of carbon rotate the beam of polarized light to the right, others to the left—are "optically active," as they are termed. Pasteur had pointed out that this is possible only in compounds in which there is some kind of asymmetry. Van't Hoff showed in what the asymmetry consisted. He showed that all optically active compounds of carbon then known contain a carbon atom in combination with four different atoms or groups, and the same holds true to-day. Such a carbon atom is known as an "asymmetric carbon atom," and thus arose the theory of the "asymmetric tetrahedral carbon atom," ​which has been the philosophy of organic chemistry for the past thirty-five years, and the guiding thought in practically all of the best work in organic chemistry from 1874 to the present time.
The second great work of Van't Hoff had also to do primarily with the chemistry of carbon. In 1867 the Norwegian physicist, Guldberg, and his son-in-law Waage, the chemist, both of Christiania, announced the law of the effect of mass or quantity on chemical reaction—the law of mass action. This was published in the "Announcements" of the University of Christiania, and very little attention was paid to it for some time. Guldberg and Waage applied their law to comparatively few reactions.
Van't Hoff, shortly after the publication of his brief paper of eleven pages in Dutch, on "The Arrangement of the Atoms in Space," took up experimentally the study of the velocities of chemical reactions and the conditions of chemical equilibria, from the standpoint of the law of mass action. He, his assistants and students, carried out an elaborate series of investigations in which the law of mass action was applied to a large number of chemical reactions, and shown to hold. The results of this work were published in French, under the French equivalent of "Studies in Chemical Dynamics." In this work the whole subject of chemical dynamics and chemical equilibrium was placed upon a scientific basis, and for the first time.
The third and greatest work of Van't Hoff had to with the relation between solutions and gases. Through his colleague—the botanist, De Vries, the attention of Van't Hoff was called to the osmotic pressure measurements that had been made by the botanist Wilhelm Pfeffer. A comparison of the results obtained by Pfeffer with the gas pressures exerted by gases containing the same number of gaseous molecules in a given volume that the solution contained dissolved molecules in the same volume, showed that the gas-pressure was exactly equal to the osmotic pressure-—in a word, the laws of gas-pressure apply to the osmotic pressure of solutions.
Van't Hoff showed that the laws of gas-pressure apply to the osmotic pressure of solutions of non-electrolytes, i. e., those substances whose aqueous solutions do not conduct the current. He also pointed out that the laws of gas-pressure do not apply to the osmotic pressure of a single electrolyte—a single acid, base or salt. Arrhenius explained the apparent discrepancy in the case of electrolytes by means of the theory of electrolytic dissociation, which says that acids, bases and salts in aqueous solution are broken down into charged parts or ions.
The question arises why is it so important to have shown that the laws of gas-pressure apply to the osmotic pressure of solutions? We know more about matter in the gaseous state than in any other state of aggregation. We can deal with gases from the standpoint of the ​only exact branch of science—mathematical physics—and Van't Hoff showed that we can deal with solutions in the same manner.
This raises the further question why is it so important to have a satisfactory theory as to the nature of solutions? A moment's thought will furnish the answer. The whole science of chemistry is a branch of the science of solutions. Similarly, the biological sciences are dependent upon solution for their existence, and geology, in dealing both with the sedimentary and the igneous rocks, is vitally concerned with solution in the broader sense of that term. There are, then, few branches of natural science that are not dependent upon solution for their existence.
Van't Hoff made a number of other contributions to science, second in importance only to those named above. He told us what is meant by "Solid Solutions." His last work was an experimental study of the conditions under which the great salt beds at Stassfurth were laid down from a desiccating inland sea.
Van't Hoff published a number of books on physical chemistry but it would lead us too far here to discuss them in any detail.
The writer knew Van't Hoff in the relation of student to teacher. He was one of the most modest, frank, honest and unselfish of men. He lived to see his work properly understood and appreciated. He was elected a member of most of the learned societies and academies in the world. He was awarded the first Nobel prize in chemistry in 1901.
The name of Van't Hoff will undoubtedly go down in the history of science along with those of the very greatest—with Maxwell and Sir J. J. Thomson; with Laplace and Pasteur, and with Helmholtz and Lorentz.