ASSAULT ON ATOMS COMPTON 291 



Because it carries this unit of electric charge, which seems to be an 

 indivisible unit, these beta particles were called electrons^ and by 

 that name they have become familiar. 



These electrons have been weighed, too, and their weight is found 

 to be very small indeed. The atom of hydrogen is the smallest atom 

 we know, and as we have seen, it is a very tiny thing. But an elec- 

 tron weighs only 1/1845 as much as does a hydrogen atom. Thus 

 we were correct in guessing that the beta particle which made the 

 faint trail was much smaller than the alpha particle that made the 

 broad bright streak on an earlier photograph. 



The electron is indeed one of the components of which the atom is 

 built. We can in fact count the number of electrons that each atom 

 has. Hydrogen has 1 electron, helium 2, lithium 3, and so on. Oxy- 

 gen has 8 electrons in each atom, iron 26, and uranium, the heaviest 

 atom of all, has 92 electrons. 



THE NUCLEUS AND THE PROTON 



But this is only a part of the story. The electrons are all par- 

 ticles of negative electricity. The atom itself is electrically neutral, 

 and must therefore have in it some positive electricity to neutralize 

 the negative electrons. If time were available, I should describe for 

 you the beautiful experiments carried out by Rutherford and Aston 

 in Cambridge, Dempster at the University of Chicago, and others, 

 which have shown that this positive electricity is concentrated in a 

 very small nucleus, which though much smaller in size that the atom 

 has yet nearly all the atom's weight. 



The careful experiments of Dempster and Aston have shown that 

 the weights of the nuclei of the various atoms, such as oxygen, nitro- 

 gen, sodium, and the rest, are whole multiples of a unit which is 

 nearly equal to the weight of the hydrogen nucleus. This suggested 

 that the various atomic nuclei are built up of hydrogen nuclei. This 

 idea was supported by the fact that the electric charge carried by 

 the various atomic nuclei is always an integral multiple of the 

 charge carried by hydrogen nucleus. 



Many attempts have been made to make one element out of an- 

 other. This is in fact the old problem of alchemy, to make gold out 

 of lead. The first success was got by Rutherford. He didn't get 

 gold out of lead ; but he did get hydrogen out of nitrogen and out of 

 aluminum and other elements. 



The experiment can best be shown using again our cloud expan- 

 sion apparatus, as has been done for example by our fellow member, 

 Professor Harkins. Plate 2, Figure 2 shows a group of alpha 

 particles shooting through nitrogen gas. Most of them go straight 

 102992—32 20 



