168 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1915. 



variation in the chemical properties of elements when arranged in 

 order of increasing atomic weight. This empirical generalization 

 has exercised a Avide 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 im- 

 portant 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 de- 

 velopment of organic chemistry. 



While the chemist was busy disentangling the elements, determin- 

 ing their relative atomic weights and studying their possible combina- 

 tions, the physicist had not been idle. The idea that a gas consisted 

 of a large number of molecules in swift but irregidar movement 

 had been tentatively advanced at various times to explain some of 

 the properties of gases. These conceptions were independently re- 

 vived and developed in great detail by the genius of Clausius and 

 Clerk Maxwell about the middle of the last century. On their the- 

 ory, now known as the kinetic or dynamical theory of gases, the mole- 

 cules of a gas are supposed to be in continuous agitation colliding 

 with each other and with the w^alls 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 tlie gas on the walls 

 of the inclosure. This theory was found to explain in a simple and 

 obxaous 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 interdiffusion 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 wath a high-powder 

 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 funda- 

 mental property of small particles in liquids. The whole question has 

 been investigated in recent years with great ability and skill by Per- 

 rin. 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 



