This volume presents the contributions delivered at the "Josef-Loschmidt-Sympo sium," which took place in Vienna, June 25-27, 1995. The symposium was arranged to honor Josef Loschmidt one hundred years after his death (8 July 1895), to evaluate the sig nificance of his contributions to chemistry and physics from a modem point of view and to trace the development of scientific fields in which he had done pioneering work. Loschmidt is widely known for the first calculation of the size of molecules (1865/66), which also led to values for the number of molecules in unit gas volume and for the mass of molecules. With critical analyses of problems in statistical physics he made important contributions to the development of that field, "Loschmidt's paradoxon" continuing to be a point of departure for present day studies and discussions. For decades there was little awareness that Loschmidt was a pioneer in organic struc tural chemistry. Only in recent years has Loschmidt's first scientific publication "Chemis che Studien I", published in 1861, become more widely known and it is now recognized that with his ideas on the structure of organic molecules he was greatly ahead of the chemists of that time. The papers in these proceedings are arranged in three sections: l. Organic structural chemistry (Chapters 1-12). 2. Physics and physical chemistry (Chapters 13-26). 3. Loschmidt's biography, Loschmidt's world (Chapters 27-33).

This volume presents the contributions delivered at the "Josef-Loschmidt-Sympo sium," which took place in Vienna, June 25-27, 1995. The symposium was arranged to honor Josef Loschmidt one hundred years after his death (8 July 1895), to evaluate the sig nificance of his contributions to chemistry and physics from a modem point of view and to trace the development of scientific fields in which he had done pioneering work. Loschmidt is widely known for the first calculation of the size of molecules (1865/66), which also led to values for the number of molecules in unit gas volume and for the mass of molecules. With critical analyses of problems in statistical physics he made important contributions to the development of that field, "Loschmidt's paradoxon" continuing to be a point of departure for present day studies and discussions. For decades there was little awareness that Loschmidt was a pioneer in organic struc tural chemistry. Only in recent years has Loschmidt's first scientific publication "Chemis che Studien I", published in 1861, become more widely known and it is now recognized that with his ideas on the structure of organic molecules he was greatly ahead of the chemists of that time. The papers in these proceedings are arranged in three sections: l. Organic structural chemistry (Chapters 1-12). 2. Physics and physical chemistry (Chapters 13-26). 3. Loschmidt's biography, Loschmidt's world (Chapters 27-33).

6 kehrten Verhiiltnis ihrer Halbwertszeiten, wenn die Reihe sich im inneren radio- aktiven Gleichgewicht befindet. Das Produkt aus der Zahl der vorhandenen Atome und der Zerfallskonstante (die der reziproken Halbwertszeit proportional ist) muB ja im stationiiren Zustand fiir jedes Glied den gleichen Wert haben. Daher ist die Gesamtaktivitiit eines kurzlebigen Gliedes einer Reihe (z. B. des Radon) im Prinzip ebensogut meBbar wie die Gesamtaktivitiit eines liinger- lebigen Gliedes derselben Reihe (z. B. des Radiums), mit dem es im radioaktiven Gleichgewicht steht, obwohl die vorhandene Gewichtsmenge so auBerordentlich viel kleiner sein kann. Voraussetzung ist nur, daB die Halbwertszeit nicht so klein ist, daB der Stoff dem Radiochemiker "zwischen den Fingern zerrinnt". Unter dieser Voraussetzung lassen sich in der Praxis die kurzlebigen Stoffe sogar leichter messen als die langlebigen Stoffe gleicher Gesamtaktivitiit. Je kleiner niimlich die Masse der Probe ist, desto vollstiindiger tritt die Strahlung aus der Probe aus und in das MeBgeriit ein. v Die von SODDY so klar zusammengefaBpen Verhiiltnisse haben dazu gefiihrt, daB radiochemische Methoden sich zur analytischen Bestimmung von natiirlichen Radioelementen mittlerer Halbwertszeit (z. B. des Radiums; genauer: des Radiumisotops der Uranreihe) als zweckmiiBig, zur analytischen Bestimmung von Radioelementen kurzer Halbwertszeit (z. R. des Poloniums) sogar als un- entbehrlich erwiesen haben. Zur Bestimmung langlebiger Radioelemente (z. B. des Urans) haben sich radiochemische Methoden nur in Sonderfiillen durch- gesetzt. Die Besprechung dieser Methoden zur Analyse natiirlich radioaktiver Stoffe ist in den entsprechenden Abschnitten des Kapitels IX durchgefiihrt.