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      In 1859 D. Mendeleyev got a permission for a foreign trip “to improve in the sciences.” He went abroad with a properly worked out original programm of the research. The theoretical idea of the close connection between the physical and chemical characteristics of the substance became its foundation. During this period Mendeleyev especially emphasized the research of the cohesion of the particles. He supposed to study them by measuring the surface tension of the liquids (the phenomenon of capillarity) at the different temperatures.

      Dmitry Ivanovich wrote, “Being sent oversea in 1859, I studied only the capillarity, supposing to find there the clue to the solution of many physico-mathematical problems”; “… I intended to determine the interdependence between the particle volume and the cohesion”; “The measure of the solid cohesion, undoubtly, is an attribute more intrinsic than i. e. the boiling-point, and until now we have very few data about it.”

      Dmitry Mendeleyev left Petersburg without having any clear idea of a science center of Europe where he was going to work. In a month spent on travelling around different cities, he chose Heidelberg, in the well-known university of which worked R. Bunsen, G. Kirchhoff, E. Erlenmeyer and other prominent scientists.

      Having settled down in Heidelberg, Mendeleyev right away decided to establish his own laboratory, since it was impossible to carry out such “delicate experiments as capillary ones” in the laboratory, offered him by R. Bunsen. While starting to work the scientist gave a great consideration to the acquirement of good measuring instruments and their thorough study. While working in Heidelberg, studying the interdependence of the particle volume and the cohesion and studying the capillarity, D. Mendeleyev worked out the system of metrology and created the unique measuring equipment. For instance, he developed a fundamentally new instrument for the determination of the liquid density, which was later named after him, – densimeter of D. I. Mendeleyev.

      Concerning the series of works of the 1850–1860’s, connected with the research of liquids, Mendeleyev told about it at the end of his life: “Being partly disappointed, I had absolutely given up this difficult subject, where, however, I was thinking independently. It is evident because I discovered the «absolute boiling-point»”. He succeeded to determine that liquid had turned to steam under a certain temperature, which was called by him the absolute boiling-point.

      This discovery is the first important scientific achievement of Mendeleyev. Later, after the works of T. Andrews, another term firmed up in the science – “critical temperature.” However, Mendeleyev’s priority in the ascertainment of this significant phenomenon is nowadays undoubted and generally acknowledged.

      Mendeleyev’s works on the subject of capillarity, realized by him in Heidelberg, are the logical continuation of his previous research. After having analyzed the whole of the scientist’s works and plans at the end of the 1850’s, it is possible to say that he longed for constructing the general system of physico-mathematical knowledge. Obviously, as a result of his research of the specific volumes the scientist made sure that knowledge about the atom size and the positional relationship of the particles wasn’t enough for the complete explanation of chemical characteristics of substances. He came to a conclusion that they should be supplemented with the characteristics, which were defining the force of interaction of the particles. Mendeleyev tried to work out the main regulations of a special theoretical discipline – molecular mechanics, which rests upon the three values: weight, volume and the force of interaction of the particles (molecules).

      Maria Dmitrievna Mendeleyeva (nee Kornilyeva; 1793–1850), the mother of D. I. Mendeleyev. Unknown painter. Oil painting

      Ivan Ivanovich Mendeleyev (1783–1847), the father of D. I. Mendeleyev. Unknown painter. Oil painting

      The building of Tobolskaya gymnasium, where D. I. Mendeleyev was studying in 1841-1849

      Gymnasium attached to the lycee de Richelieu in Odessa, where D. I. Mendeleyev was teaching in 1855-1856

      The attempt to construct the molecular mechanics is very interesting. It is an example of the orientation of the scientist’s works to the significant theoretical generalizations. Though today this idea is only of a historical importance, nevertheless, it describes perfectly the independent approach of the scientist to the solution of the problems of the substance structure. In the middle of the 19^th century it hadn’t been generally acknowledged yet and it had been supported only by individual scientists in different countries. The molecular theory started to be generally acknowledged only after the International chemical congress in Carlsruhe in 1860.

      Participation in the International chemical congress, which took place on September, 3^rd-5^th of 1860, became for Mendeleyev one of the most bright events of that year, which influenced greatly upon his choice of scientific interests during the following years. Mendeleyev came to the congress as a member of the delegation of Russian chemists, where were N. N. Zinin, A. P. Borodin, L. P. Shishkov, etc. During the congress’s work Dmitry Mendeleyev got acquainted with many prominent scientists of Europe. Those were J.– B. Dumas, Ch. Wurza and S. Cannizzaro, G. Rosko, etc. He continued communicating with them later.

      It is difficult to overestimate the meaning of the International chemical congress in the history of chemistry. There was accepted the common system of atomic weights and were defined the conceptions of the molecule and atom.

      As stated above, the scientific conceptions, which had been generally acknowledged at the congress of 1860, appeared in Mendeleyev’s research even before; the fundamentals of the molecular theory, as well as the principles of defining the molecular weight and density, were delivered by him at his lectures.

      At the beginning of the 1860’s another important event took place in the life of D. I. Mendeleyev. On April, 29^th of 1862 there was a wedding of Dmitry Mendeleyev and Theozva Nikitichna Leshchova, the stepdaughter of Pyotr Petrovich Ershov, the authour of the fairy-tale “Konyok-Gorbunok” (“The gibbous horse”). The wife was six years older than her husband. Their first-born son was Volodya, then daughter Olga was born. But this marriage wasn’t a happy one. Active

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