Faraday narrowly misses the discovery of the doctrine of conservation--Carnot's belief that a definite quantity of work can be transformed into a definite quantity of heat--The work of James Prescott Joule--Investigations begun by Dr. Mayer--Mayer's paper of 1842--His statement of the law of the conservation of energy--Mayer and Helmholtz--Joule's paper of 1843--Joule or Mayer--Lord Kelvin and the dissipation of energy-The final unification. CHAPTER IX. THE ETHER AND PONDERABLE MATTER James Clerk-Maxwell's conception of ether--Thomas Young and "Luminiferous ether,"--Young's and Fresnel's conception of transverse luminiferous undulations--Faraday's experiments pointing to the existence of ether--Professor Lodge's suggestion of two ethers--Lord Kelvin's calculation of the probable density of ether--The vortex theory of atoms--Helmholtz's calculations in vortex motions--Professor Tait's apparatus for creating vortex rings in the air---The ultimate constitution of matter as conceived by Boscovich--Davy's speculations as to the changes that occur in the substance of matter at different temperatures--Clausius's and Maxwell's investigations of the kinetic theory of gases--Lord Kelvin's estimate of the size of the molecule--Studies of the potential energy of molecules--Action of gases at low temperatures. APPENDIX A HISTORY OF SCIENCE BOOK III. MODERN DEVELOPMENT OF THE PHYSICAL SCIENCES With the present book we enter the field of the distinctively modern. There is no precise date at which we take up each of the successive stories, but the main sweep of development has to do in each case with the nineteenth century. We shall see at once that this is a time both of rapid progress and of great differentiation. We have heard almost nothing hitherto of such sciences as paleontology, geology, and meteorology, each of which now demands full attention. Meantime, astronomy and what the workers of the elder day called natural philosophy become wonderfully diversified and present numerous phases that would have been startling enough to the star-gazers and philosophers of the earlier epoch. Thus, for example, in the field of astronomy, Herschel is able, thanks to his perfected telescope, to discover a new planet and then to reach out into the depths of space and gain such knowledge of stars and nebulae as hitherto no one had more than dreamed of. Then, in rapid sequence, a whole coterie of hitherto unsuspected minor planets is discovered, stellar distances are measured, some members of the starry galaxy are timed in their flight, the direction of movement of the solar system itself is investigated, the spectroscope reveals the chemical composition even of suns that are unthinkably distant, and a tangible theory is grasped of the universal cycle which includes the birth and death of worlds.