244 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 



ties of solid and liquid bodies should be amenable to interpretation 

 by picturing the molecules and atoms in such states as existing much 

 closer to each other than in the gaseous state. The considerable 

 forces which should then be expected to exist between the atoms and 

 molecules should lead to an accounting for the properties of incom- 

 pressibility, cohesion, etc., which characterize solids and liquids. 



The atomic theory of matter has been confirmed by remarkable 

 experiments such as those of M. Jean Perrin which permitted him to 

 measure the number and weight of the divers atoms within a cubic 

 centimeter. I will indicate hefe merely that the weight of an atom 

 is extraordinarily small, yet the number per cubic centimeter is al- 

 ways enormous under ordinary conditions. 



Without going further into the development of the atomic theory, 

 we will merely add that in physics as in chemistry, the hypothesis, 

 according to which all bodies are composed of molecules which them- 

 selves are built up of divers combinations of elementary atoms, has 

 proved itself very fruitful and should be considered a good model of 

 reality. 



The physicists, however, were not content to stop here. They 

 wished to proceed further — to know the structure of the atoms — in 

 what way the various elements themselves differed from each other. 

 In this task they were aided by the progress which had come in their 

 knowledge of electrical phenomena. From the very beginning of 

 the researches into the study of electrical processes it had seemed 

 useful to treat electricity as a fluid — to suppose, for example, that 

 the electric current passing through a wire was the flowing of some 

 fluid through it. But as you know, there are apparently two kinds 

 of electricity, positive and negative. Consequently it was natural to 

 postulate the existence of two fluids, a positive fluid and a negative 

 fluid. We may picture these fluids in two different ways: It is 

 possible to suppose the fluid to be a continuous substance occupying 

 uniformly the region it occupies ; or better, to suppose it to be formed 

 of clouds of small corpuscles, each corpuscle being a small ball of 

 electricity. Experimental evidence has decided in favor of the 

 latter. For some 30 years we have known that negative electricity 

 is composed of small corpuscles, all identical and of extraordinarily 

 small masses and electrical charges. These corpuscles of negative 

 electricity have been designated electrons. It has become possible 

 to eject these electrons out from matter and then study their 

 behavior as free electrons in a vacuum. It has been shown thus that 

 their motions are such as would be predicted by the laws of classical 

 mechanics for small electrified particles. The study of the behavior 

 of these small particles in electric and magnetic fields has led to the 

 measurement of their size and electrical charge. These are extremely 



