gust, 1879, Mendeleef pointed out the relations which existed between the density and the atomic weights of the elements; these were subsequently more fully examined by Lothar Meyer, and are embodied in the well-known curve in his Modern Theories of Chemistry. Similar relations have been observed in certain other properties, such as ductility, fusibility, hardness, volatility, crystalline form, and thermal expansion; in the refraction equivalents of the elements, and in their conductivities for heat and electricity; in their magnetic properties and electro-chemical behavior; in the heats of formation of their haloid compounds; and even in such properties as their elasticity, breaking stress, etc." While one may be readily inclined and many have been led to look for a connection between the periodic law and theories of the unitary origin of matter, Mendeleef has not allowed his studies in the subject to be embarrassed by any such prepossession. He said in his Faraday lecture: "The periodic law, based as it is on the solid and wholesome ground of experimental research, has been evolved independently of any conception as to the nature of the elements; it does not in the least originate in the idea of a unique matter; and it has no historical connection with that relic of the torments of classical thought, and therefore it affords no more indication of the unity of matter, or of the compound nature of the elements, than do the laws of Avogadro or Gerhardt, or the law of specific heats, or even the conclusions of spectrum analysis." The periodic law is developed in the author's Principles of Chemistry, which was first published in 1869, and appeared in a fourth edition, after a thorough revision, with many important additions and modifications, in 1882.
In a lecture before the Royal Institution in 1889, Mendeleef sought to apply a broader generalization and to discover a harmonious law regulating both chemical and astronomical phenomena. The immediate object of the lecture was to show that, starting from Newton's third law of motion, it is possible to preserve to chemistry all the advantages arising from structural teaching, without being obliged to build up molecules in solid and motionless figures, or to attempt to ascribe to atoms definite limited valencies, directions of cohesion, or affinities. He supposed that harmonious order reigns in the invisible and apparently chaotic motions of the universe, reaching from the stars to the minutest atoms, which is commonly mistaken for complete rest, but which is really a consequence of the conservation of dynamic equilibrium that was discovered by Newton, and has been traced by his successors as relative immobility in the midst of universal and active movement. The unseen world of chemical changes was regarded as analogous to the invisible world of the heavenly bodies," since
