the general scientific reader any idea whatever of their nature and theory.  Here, however, they are explained with clearness and elegance, and their bearing on the undulatory theory of light is distinctly shown.  As other instances of most admirable exposition, we may call attention to the paragraphs on crystallization, on the atomic theory, on isomerism and allotropism, on diamagnetism, magnetic induction, and electric “currents,” on the sources of heat, on the chemical and thermal spectra, on the correlation and equivalence of the forces, on the theory of ozone, on the exceptional expansion of water and the supposed complexity of its atom, on the structure of flame, on the constitution of salts, on the colloid condition of matter, on types and compound radicles, on the dynamics of vegetable growth and the production of animal power, and, above all, to the passage which describes the phenomena of latent heat.  Throughout, in treating of these subjects, the author’s felicity of exposition never fails him.  The most difficult phenomena are rendered perfectly easy of comprehension, and their mutual relations are not left out of account.  Each set of facts is treated, not as forming an isolated body of truth, but as an integral portion of the complex and logically indivisible universe.  In this respect Dr. Youmans’s work is far superior to the recent production of Dr. Hooker, in which, for example, the mere existence of such a doctrine as that of the correlation of forces is grudgingly noticed, and its ultimate significance entirely overlooked.

Far different is Dr. Youmans’s treatment of the same doctrine.  Indeed, we think that the chapters on chemical physics form the most interesting portion of his work, and their value consists chiefly in the constant reference to the modern ideas of force which pervades them.  In a work intended for the education of youth, such a feature cannot be too highly praised.  It is time that the old material superstitions about force were eradicated from men’s minds, and as far as possible from their language.  It is already more than half a century since Count Rumford demonstrated the immaterial nature of heat, and Young established the undulatory theory of light,—­ideas which had germinated two hundred years ago in the lofty minds of Huygens and Hooke.  Since then have been discovered the polarization and interference of heat, the triple constitution of the solar ray, the identity of magnetism and electricity, the polar nature of chemical affinity, the optical polarities of crystals, and the interaction of magnetism and light.  Since then the once meagre and fragmentary science of physics has become one of the grandest and richest departments of human thought; and the illustrious names of Helmholtz, Joule, and Mayer, of Grove, Faraday, and Tyndall, may be fitly named beside those of the leading thinkers of past ages.  The physical forces are no longer to be looked upon as inscrutable material entities,—­forms of matter imponderable,