[I] This is the more curious, as Ampere's hypothesis that vibrations of molecules, causing and caused by vibrations of the ether, constitute heat, is discussed. See vol. ii. p. 587, 2nd ed. In the _Philosophy of the Inductive Sciences_, 2nd ed., 1847, p. 239, Whewell remarks, _a propos_ of Bacon's definition of heat, 'that it is an expansive, restrained motion, modified in certain ways, and exerted in the smaller particles of the body;' that 'although the exact nature of heat is still an obscure and controverted matter, the science of heat now consists of many important truths; and that to none of these truths is there any approximation in Bacon's essay.' In point of fact, Bacon's statement, however much open to criticism, does contain a distinct approximation to the most important of all the truths respecting heat which had been discovered when Whewell wrote. [J] Perhaps I ought rather to say Button's axiom. For that great naturalist and writer embodied the principles of sound geology in a pithy phrase of the _Theoris de la Terre_: 'Pour juger de ce qui est arrive, et meme de ce qui arrivera, nous n'avons qu'a examiner ce qui arrive.' THOMAS H. HUXLEY'S WORKS. SCIENCE AND CULTURE, AND OTHER ESSAYS. 12mo. Cloth, $1.50. THE CRAYFISH: AN INTRODUCTION TO THE STUDY OF ZOOeLOGY. With 82 Illustrations. 12mo. Cloth, $1.75. SCIENCE PRIMERS: INTRODUCTORY. 18mo. Flexible cloth, 45 cents. MAN'S PLACE IN NATURE. 12mo. Cloth, $1.25. ON THE ORIGIN OF SPECIES. 12mo. Cloth, $1.00. MORE CRITICISMS ON DARWIN, AND ADMINISTRATIVE NIHILISM. 12mo. Limp cloth, 50 cents. MANUAL OF THE ANATOMY OF VERTEBRATED ANIMALS. Illustrated. 12mo. Cloth, $2.50. MANUAL OF THE ANATOMY OF INVERTEBRATED ANIMALS. 12mo. Cloth, $2.50. LAY SERMONS, ADDRESSES, AND REVIEWS. 12 mo. Cloth, $1.75. CRITIQUES AND ADDRESSES. 12mo. Cloth, $1.50. AMERICAN ADDRESSES; WITH A LECTURE ON THE STUDY OF BIOLOGY. 12mo. Cloth, $1.25. PHYSIOGRAPHY: AN INTRODUCTION TO THE STUDY OF NATURE. With Illustrations and Colored Plates. 12mo. Cloth, $2.50. HUXLEY AND YOUMANS'S ELEMENTS OF PHYSIOLOGY AND HYGIENE. By T.H. HUXLEY and W.J. YOUMANS. 12mo. Cloth, $1.50. JOHN TYNDALL'S WORKS. ESSAYS ON THE FLOATING MATTER OF THE AIR, in Relation to Putrefaction and Infection. 12mo. Cloth, $1.50. ON FORMS OF WATER, in Clouds, Rivers, Ice, and Glaciers. With 35 Illustrations. 12mo. Cloth, $1.50. HEAT AS A MODE OF MOTION. New edition. 12mo. Cloth, $2.50. ON SOUND: A Course of Eight Lectures delivered at the Royal Institution of Great Britain. Illustrated. 12mo. New edition. Cloth, $2.00. FRAGMENTS OF SCIENCE FOR UNSCIENTIFIC PEOPLE. 12mo. New revised and enlarged edition. Cloth, $2.50. LIGHT AND ELECTRICITY. 12mo. Cloth, $1.25. LESSONS IN ELECTRICITY, 1875-'76. 12mo. Cloth, $1.00.