106 THE POPULAR SCIENCE MONTHLY 



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 explana- 

 tion 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 development 

 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 irregular movement had 

 been tentatively advanced at various times to explain some of the prop- 

 erties 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 djmamical 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 fun- 

 damental 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 inter- 

 esting 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 disturb- 

 ances, 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 ex- 

 cursions 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 



