OSTWALD'S “Hand- und Hilfsbuch” has changed almost beyond recognition since it was first issued in 1893. It was then a comparatively small book, and the reviewer remembers vividly how as a student he used it as a text-book of scientific German. The second edition, issued in 1902, called for the services of a collaborator in the person of Prof. Luther; and now, fifteen years after the appearance of the third edition, the issue by Prof. Drucker of a fourth edition has only been possible with the aid of a long list of colla borators, who have contributed complete sections on optical measurements, X-ray measurements, radio active measurements, etc. Including the indexes, the whole volume now covers more than 800 pages, and it would have been still larger but for the fact that, in the opinion of the responsible editor, the appearance of the “Kolloidchemische Praktikum” of Wo. Ostwald has rendered it no longer necessary to add a section on the technique of colloid chemistry. In the same way the revision of the “Physikalisch-Chemischen Tabellen” of Roth and Scheel has made it possible to omit most of the numerical data. The details of the practical course at Leipzig (which includes fourteen electrical exercises, but only one optical experiment, with a sodium lamp as the only light-source) have, however, been retained as an appendix.