These ideas have also been more or less successfully applied to the study of the composition of inorganic compounds. The assistance organic chemistry has given to the general subject is incalculable. Finally, it may be said, that while in the nature of the case our ideas of structure in organic compounds cannot be regarded as proved, or as not subject to possible future modifications, we have, at least, a consistent theory and good working hypothesis. A homely illustration of our present ideas may be drawn from the modern high city building. The skeleton of this building is made of iron, about which are grouped the brick, stone, wood, and other materials to form a complete building. So the organic body is built on a chain or frame-work or skeleton of carbon atoms, about which are grouped the atoms of hydrogen, oxygen, and nitrogen, or radicle compounds thereof. It is not possible here to even name some of the more eminent workers who for a quarter of a century have contributed to our knowledge of organic chemistry. This branch of chemistry has been the vogue, and has been pushed almost to the limit of possibility since 1875. Many almost unexplored fields still remain, but chemists recognize the fact that in theory and practice organic chemistry has reached a high degree of perfection, and they are returning to continue the researches in other fields which have for so long been almost neglected. IV. ANALYTICAL CHEMISTRY. No branch of chemical science has a more general interest for the public than that which relates to the determination of the materials of which bodies are composed, and the proportions in which they exist. At the beginning of the century considerable progress had been made in this branch of knowledge by the researches of Boyle (1626–1691), Hoffmann, Margraff (1709–1780), Scheele and Bergmann (1735–1784). Berzelius, as has already been mentioned, had added a new and valuable factor to chemical analysis by the development of the blowpipe, and in the early part of the century mineral analysis was still further advanced by Klaproth (1743–1817), Rose (1798–1873), and many others. No one man did so much to advance this branch of chemical science as Fresenius (1818–1897). He collated and proved all the proposed methods of analysis, both qualitative and quantitative, and out of a confused mass of material formed a logical system of procedure, which has proved invaluable to the progress of chemical science in all its branches. The volumetric methods of analysis, which save so much time and labor without sacrificing accuracy, were developed by Gay-Lussac, Vauquelin (1763–1879), Mohr (1806–1879), Volhard, Sutton, Fehling, and Liebig. The methods of gas analysis have been worked out chiefly by Bunsen, ably assisted by Winkler and Hempel.