ties of elements when arranged in order of increasing atomic weight. This empirical generalization has exercised a wide influence on the development of chemistry, and the periodic law has been considered by many to indicate that all the atoms are composed of some elementary substance or protyle. It is only within the last few years that our knowledge of atoms has reached a stage to offer a reasonable explanation of this remarkable periodicity.
Time does not allow me to more than refer in passing to the important contributions of Le Bel and van' t Hoff to the structure of complex molecules, and the arrangements of the atoms in space, which has exercised such a wide and important influence on the development of organic chemistry.
While the chemist was busy disentangling the elements, determining their relative atomic weights and studying their possible combinations, the physicist had not been idle. The idea that a gas consisted of a large number of molecules in swift but irregular movement had been tentatively advanced at various times to explain some of the properties of gases. These conceptions were independently revived and developed in great detail by the genius of Clausius and Clerk Maxwell about the middle of the last century. On their theory, now known as the kinetic or dynamical theory of gases, the molecules of a gas are supposed to be in continuous agitation, colliding with each other and with the walls of the containing vessel. Their velocity of agitation is supposed to increase with temperature, and the pressure is due to the impact of the molecules of the gas on the walls of the enclosure. This theory was found to explain in a simple and obvious way the fundamental properties of gases, and has proved of great importance in molecular theory. The idea that atoms must be in brisk and turbulent motion is strongly supported by the well-known property of the inter-diffusion of gases and also of liquids, and in recent years has received practically a direct and concrete proof from the study of a very interesting phenomenon included under the name "Brownian Motion." The English botanist, Brown, in 1827 discovered that small vegetable spores immersed in a liquid appeared to be in continuous motion when viewed with a high power microscope. This motion of small particles in liquids was at first supposed to be a result of temperature disturbances, but at the close of the last century the Brownian movement was shown to be a fundamental property of small particles in liquids. The whole question has been investigated in recent years with great ability and skill by Perrin. He examined in detail the state of equilibrium and of motion of minute particles in suspension in liquids. The excursions due to the Brownian movements depend mainly on the size of the particles, although influenced to some extent by the nature of the liquid. Small spheres of the size required can be produced by a variety of methods. One of the simplest used by Perrin is to allow a solution
