292 ANNUAL, EEPORT SMITHSONIAN INSTITUTION, 1931 



to the end of the path, and this is remarkable, for each alpha passes 

 right through tens of thousands of nitrogen atoms before its flight 

 is stopped. But here we see a really surprising occurrence. The 

 alpha particle dives into a nitrogen atom, and out of it emerges a 

 smaller particle, which goes out leaving a thin straight trail. The 

 nitrogen nucleus with the alpha particle now attached moves heavily 

 along in a different direction. The alpha particle has served as a 

 hammer to knock a hj^drogen nucleus out of a nitrogen atom. 



Similar experiments have been done with- many other elements, 

 and most of the lighter ones have thus been disintegrated, expelling 

 always a hydrogen nucleus. Thus we may take this nucleus, like the 

 electron, as a component of which the various atoms are built. We 

 give to the hydrogen nucleus now the name of proton^ i. e., the origi- 

 nal or fundamental thing. Out of protons and electrons we believe 

 all the 92 different kinds of atoms are built. 



HOW THE ATOM IS BUILT 



Old Ptolemy, the ancient Greek astronomer, knew that there was 

 a sun and a moon, the earth, and the planets, but he didn't know 

 what the solar sj'stem is. When Copernicus and Galileo showed, 

 however, that there is a sun, around which revolve planets in definite 

 orbits, then men felt that they had become acquainted with their 

 world. So, though we have found the parts of which the atom is 

 made, we really don't know the atom until we know how these parts 

 are put together. 



Perhaps the best way to find out how something is made is to 

 look at it. If it is something like a watch, which we can hold in 

 our hands, this is comparatively easy. If it is the cell structure 

 of a muscle that we wish to examine, we put it under a microscope. 

 But some things are too small to see, even in a microscope. By using 

 ultra-violet light of wave length shorter than ordinary light, we can 

 photograph such things as typhoid bacilli with increased sharpness. 

 But atoms are too small even for this. 



Now, X rays have a wave length onh'' a ten-thousandth that of 

 light, and if we could use them in a microscope it should be pos- 

 sible for us to observe even the tiny atoms. Unfortunatel3% we can 

 not make lenses that will refract X rays, and even if we could, 

 our eyes are not sensitive to X rays. So it would seem that we 

 shall never be able to see an atom directly. 



It is nevertheless possible for us in the laborator}" to get by more 

 round-about methods precisely the same information about an atom 

 that we should if we could look at it with an X-ray microscope. 

 I have spent a large part of the last 16 years trying to find what 

 the atom looks like, and it has become something of a game with me. 



