XXVI.—DMITRI IVANOWITSH MENDELEEFF. DMITRI IVANOWITSH MENDELEEFF was born on February 7 (N.S.), 1834, at Tobolsk, in Siberia. He was the seventeenth and youngest child of Ivan Paolowitsh Mendeleeff, Director of the Gymnasium at that place. Soon after the birth of Dmitri his father became blind, and was obliged to resign his position, and the family became practically dependent upon the mother, Maria Dmitrievna Mendeleeva—a woman of great energy and remarkable force of character. She established a glass-works at Tobolsk, the management of which for many years devolved entirely upon her, and on the profits of which she brought up and educated her large family. The story of Mendeleeff's youth is given in the preface to his great work “On Solutions,” which he dedicated to the memory of his mother in a passage of singular beauty and power. Having passed through the Gymnasium at Tobolsk, Mendeleeff, at the age of sixteen, was sent to St. Petersburg, with the intention that he should study chemistry at the University, under Zinin. He was, however, transferred to the Pedagogical Institute, the aim of which was to train teachers for the District or Governmental Gymnasiums throughout the Empire. The Institute (which was abolished in 1858) was established in the same building as the University, and was divided into two Faculties—Historico-philological and Physico-mathematical. Mendeleeff attached himself to the natural sciences, and thus came under the influence of Woskresenky in chemistry, of Emil Lenz in physics, of Ostrogradsky in mathematics, of Ruprecht in botany, of F. Brandt in zoology, of Kutorga in mineralogy, and of Sawitsh in astronomy, most of whom were Professors of the same sciences in the University. Whilst at the Institute he wrote his first paper on a “Isomorphism,” and on the termination of his course of instruction he was appointed to the Gymnasium at Simferopol, in the Crimea. During the Crimean war he was transferred to one of the Gymnasiums at Odessa, and in 1856 he was admitted to the degree of Magister Chemiæ of the Physico-mathematical Faculty of the University of St. Petersburg, and was made Privat-Docent in the University. Even at this early period of his career we find Mendeleeff speculating on the great problems with which his name is inseparably connected. The relations between the specific gravities of substances and their molecular weights had begun to attract increased attention. Kopp had just published the first instalment of that long and laborious series of experimental observations which constitutes the real foundation of all our knowledge concerning the specific volumes of liquids, when the young Siberian philosopher laid a number of theses on problems relating to specific volumes before the Physico-mathematical Faculty of the University. He pointed out that magnetic elements have smaller specific volumes than diamagnetic elements. He also showed that Avogadro's supposition, that electro-positive elements have larger specific volumes than electro-negative elements, was in accordance with the greater number of well-established facts. When we remember how slowly, in spite of the powerful advocacy of Williamson, the ideas of Laurent and Gerhardt and what came to be known as the modern French school, found favour in this country, it is remarkable, as indicating the radical and progressive character of his mind, and the keenness of his mental vision, to fina Mendeleeff, as far back as 1856, insisting that to Gerhardt was due the best mode of determining the chemical molecule; that the molecule of oxygen was expressed by the symbol O2; those of arsenic and phosphorus by As4 and P4 respectively; that of alcohol by ; and that of ether by O. Mendeleeff's researches on specific volumes were begun in 1855, and were continued, with intermissions, down to [1870; but part only of the work has been published. In 1859, Mendeleeff obtained permission from the Minister of Public Instruction to travel, and repaired to Heidelberg, where he established a small private laboratory, and occupied himself with the determination of the physical constants of chemical compounds. He returned home in 1861, and in 1863 was named Professor of Chemistry at the Technological Institute of St. Petersburg. In 1866 he became Professor of Chemistry at the University, and was made Doctor of Chemistry after a public defence of his dissertation "On the Combinations of Water with Alcohol."He is now Emeritus Professor, and delivers annually a course of lectures on general chemistry.