XXII. THE THEORY OF SOUND IN ITS RELATION TO MUSIC. By Professor Pietro Blaserna. With numerous Illustrations. $1.50.
XXIII. STUDIES IN SPECTRUM ANALYSIS. By J. Norman Lockyer, F. R. S. With Six Photographic Illustrations of Spectra, and numerous Engravings on Wood. $2.50.
XXIV. A HISTORY OF THE GROWTH OF THE STEAM-ENGINE. By Professor R. H. Thurston. With 163 Illustrations. $2.50.
XXV. EDUCATION AS A SCIENCE. By Alexander Bain, LL. D. $1.75.
XXVI. STUDENTS' TEXT-BOOK OF COLOR, OR, Modern Chromatics. With Applications to Art and Industry. By Professor Ogden N. Rood, Columbia College. New edition. With 130 Illustrations. $2.00.
XXVII. THE HUMAN SPECIES. By Professor A. de Quatrefages, Membre de l'Institut. $2.00.
XXVIII. THE CRAYFISH: an Introduction to the Study of Zoölogy. By T. H. Huxley, F. R. S. With 82 Illustrations. $1.75.
XXIX. THE ATOMIC THEORY. By Professor A. Wurtz. Translated by E. Cleminshaw, F. C. S. $1.50.
XXX. ANIMAL LIFE AS AFFECTED BY THE NATURAL CONDITIONS OF EXISTENCE. By Karl Semper. With Two Maps and 106 Woodcuts. $2.00.