290 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 31 



Molecules and atoms are very tiny things; but there are so many 

 of them that they make up the world in which we live. 



The story is told of Lord Kelvin, a famous Scotch physicist of the 

 last century, that after he had given a lecture on atoms and mole- 

 cules, one of his students came to him with the question, " Professor, 

 what is your idea of the structure of the atom ? " " What," said 

 Kelvin, "the structure of the atom? Why, don't you know, the 

 very word ' atom ' means the thing that can't be cut. How, then, 

 can it have a structure? " 



" That," remarks the facetious young man, " shows the disad- 

 vantage of knowing Greek." 



Does the atom have parts ? 



THE ELECTRON 



Do 3^ou see the faint little trail at the bottom of Plate 1, Figure 4? 

 It appears to be due to something much smaller than the particle 

 which made the broad bright trail above it. If we called the one 

 an alpha particle, let us call the other a beta particle, and try to find 

 out what kind of thing it is. 



Plate 1, Figure 5 shows a large number of these beta particles, 

 that have been knocked out of air molecules by the action of X rays. 

 You can see where the X rays passed through the middle of the 

 chamber. Now every substance has its own peculiar kind of atoms. 

 Iron atoms differ from oxygen atoms, and these from atoms of 

 carbon and so on. But these beta particles are all alike, as far as we 

 can tell, and they can be knocked out of anything. Had we put into 

 the chamber fried eggs or a platinum wrist watch, the same kind of 

 beta particles would have been observed. Thus beta particles are 

 things which go to make up all kinds of matter. They are more 

 fundamental even than atoms. 



But what are these beta particles? In the first place they carry 

 an electric charge. Notice in Plate 2, Figure 1 how their trails 

 are curled up if a magnet is held near the expansion chamber. This 

 is because the moving electric charge acts like a wire carrying an 

 electric current, and the particles form the armatures of tiny electric 

 motors. 



Professor Millikan, a member of our society, spent years at the 

 University of Chicago in measuring the charge carried by one of 

 these little particles. He built himself an electroscope in which a 

 tiny drop of oil took the place of the usual gold leaf, and he would 

 catch these beta particles on his oil drop. Every particle carried 

 the same charge, he found. It was also the same charge that a 

 hydrogen ion carries when water is dissociated into oxygen and 

 hydrogen by the passage of an electric current. 



