Popular Lectures on Scientific Subjects. By H. HELMHOLTZ, Professor of Physics in the University of Berlin. First Series. Translated by E. ATKINSON, Ph. D., F. C. S. With an Introduction by Professor TYNDALL. With 51 Illustrations. 12mo. Cloth, $2.00. _CONTENTS._--On the Relation of Natural Science to Science in General.--On Goethe's Scientific Researches.--On the Physiological Causes of Harmony in Music.--Ice and Glaciers.--Interaction of the Natural Forces.--The Recent Progress of the Theory of Vision.--The Conservation of Force.--Aim and Progress of Physical Science. Popular Lectures on Scientific Subjects. By H. HELMHOLTZ. Second Series. 12mo. Cloth, $1.50. _CONTENTS._--Gustav Magnus.--In Memoriam.--The Origin and Significance of Geometrical Axioms.--Relation of Optics to Painting.--Origin of the Planetary System.--On Thought in Medicine.--Academic Freedom in German Universities. "Professor Helmholtz's second series of 'Popular Lectures on Scientific Subjects' forms a volume of singular interest and value. He who anticipates a dry record of facts or a sequence of immature generalization will find himself happily mistaken. In style and method these discourses are models of excellence, and, since they come from a man whose learning and authority are beyond dispute, they may be accepted as presenting the conclusions of the best thought of the times in scientific fields."--_Boston Traveler._ Science and Culture, and other Essays. By Professor T. H. HUXLEY, F. R. S. 12mo. Cloth, $1.50. "Of the essays that have been collected by Professor Huxley in this volume, the first four deal with some aspect of education. Most of the remainder are expositions of the results of biological research, and, at the same time, illustrations of the history of scientific ideas. Some of these are among the most interesting of Professor Huxley's contributions to the literature of science."--_London Academy._ "It is refreshing to be brought into converse with one of the most vigorous and acute thinkers of our time, who has the power of putting his thoughts into language so clear and forcible."--_London Spectator._ Scientific Culture, and other Essays. By JOSIAH PARSONS COOKE, Professor of Chemistry and Mineralogy in Harvard College. 12mo. Cloth, $1.00.