The first chapter embraces what we know regarding atoms, and gives also a definition of Energy. The various forces and energies of Nature are thereafter enumerated, and the law of Conservation is stated. Then follow the various transmutations of Energy, according to a list, for which the author is indebted to Prof. Tait. The fifth chapter gives a short historical sketch of the subject, ending with the law of Dissipation; while the sixth and last chapter gives some account of the position of living beings in this universe of Energy. B. S. _The Owens College, Manchester, August, 1873._ CONTENTS. NOTE TO THE AMERICAN EDITION, v PREFACE, vii CHAPTER I.--WHAT IS ENERGY? 1 II.--MECHANICAL ENERGY AND ITS CHANGE INTO HEAT, 23 III.--THE FORCES AND ENERGIES OF NATURE: THE LAW OF CONSERVATION, 48 IV.--TRANSMUTATIONS OF ENERGY, 87 V.--HISTORICAL SKETCH: THE DISSIPATION OF ENERGY, 131 VI.--THE POSITION OF LIFE, 154 APPENDIX I.--CORRELATION OF VITAL WITH CHEMICAL AND PHYSICAL FORCES. By JOSEPH LE CONTE, Professor of Geology and Natural History in the University of California, 171 II.--CORRELATION OF NERVOUS AND MENTAL FORCES. By ALEXANDER BAIN, Professor of Logic and Mental Philosophy in the University of Aberdeen, 205 THE CONSERVATION OF ENERGY. CHAPTER I. _WHAT IS ENERGY?_ _Our Ignorance of Individuals._ 1. Very often we know little or nothing of individuals, while we yet possess a definite knowledge of the laws which regulate communities. The Registrar-General, for example, will tell us that the death-rate in London varies with the temperature in such a manner that a very low temperature is invariably accompanied by a very high death-rate. But if we ask him to select some one individual, and explain to us in what manner his death was caused by the low temperature, he will, most probably, be unable to do so. Again, we may be quite sure that after a bad harvest there will be a large importation of wheat into the country, while, at the same time, we are quite ignorant of the individual journeys of the various particles of flour that go to make up a loaf of bread. Or yet again, we know that there is a constant carriage of air from the poles to the equator, as shown by the trade winds, and yet no man is able to individualize a particle of this air, and describe its various motions. 2. Nor is our knowledge of individuals greater in the domains of physical science. We know nothing, or next to nothing, of the ultimate structure and properties of matter, whether organic or inorganic.