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 auspices of the Western University of Pennsylvania. In the summer of 1860 he again visited Europe, spending his time while abroad mainly in studying the scientific collections, institutions, and resources of London and Paris.

During the month of August, 1861, on the recommendation of his friend President Hitchcock, of Amherst College, he was tendered the professorship of Natural Science in Wheaton College, Illinois. This he accepted, and discharged the duties of the chair for the subsequent college year. In September, 1862, he went to Albany, at the solicitation of his friend Professor C. H. Porter, to act as his substitute in the chair of Chemistry in the Albany Medical College, Professor Porter having entered the army as Assistant Surgeon of Volunteers. While thus acting, he pursued regularly his medical studies, and was graduated therefrom as a Doctor of Medicine in 1863. After a third course of chemical lectures in Albany, given in the fall of 1864, Professor Barker went to Pittsburg as Professor of Natural Science in the Western University, remaining there during one year.

In the winter of 1865-'66, while preparing to enter the service of the United States as an Assistant Surgeon of Volunteers, having been offered a commission by Dr. Quackenbush, then Surgeon-General of the State of New York, he was offered by Professor Silliman the position of Demonstrator of Chemistry in the Yale Medical College. This offer was accepted, and he entered immediately upon his duties. Early in the spring of 1866 Professor Barker wrote the first part of a text-book, intended as a new edition of Silliman's "Chemistry." In this book, the modern nomenclature and notation appeared in a textbook for the first time in this country. The theory of types was made use of as a basis of classification, and the book was used with the senior class in Yale College.

During the absence of Professor Silliman in California in 1866-'67, the entire instruction in chemistry, in the Academical Department of Yale, was given by Dr. Barker. At the commencement in 1867, he was appointed Professor of Physiological Chemistry and Toxicology in the Medical Institution of Yale College. The chemical lectures in Williams College, in the absence of an instructor in that science, were given by him in the spring of the years 1868 and 1869. In the summer of 1870 he wrote a chemical manual entitled "A Text-book of Elementary Chemistry," which was published in September. In this book it was assumed that one philosophy was broad enough for the whole of chemical science; and hence the subject was divided into four sections—Theoretical, Inorganic, Organic, and Physiological—only the first two of which were presented in the volume mentioned. It achieved a very considerable success, about ten thousand copies having been sold within the five years after its publication, and translations of it into French and into Japanese having been made. It was adopted as the text-book in the University of Tokio, Japan. In