MODERN PHYSICS 19 



tangent ; and in this manner we find how big- that nucleus 

 is. By the size of the nucleus I mean the size of that 

 portion of the atom which is impenetrable to the alpha 

 particles. It amounts to about 10-13,000,000,000,000 cen- 

 timeters. The diameter of the atom is IO' 8 . Furthermore, 

 by counting how the deflections of the alpha particles are 

 distributed around this sphere, which we can do directly 

 with the aid of zinc sulphide spread over the inside of 

 the sphere, we can obtain the 'number of alpha particles 

 deflected through any given angle. With a little analysis 

 of unquestionable correctness, we find how many unit 

 charges, positive electrons, there are in this exceedingly 

 small nucleus, and it comes out approximately to one-half 

 of the atomic weight. 



I now come to another extraordinary discovery. This 

 one does not merely tell us approximately how many elec- 

 trons there are in the nucleus but it tells us exactly how 

 many there are, and the result checks with the number 

 obtained by the foregoing approximate method. This 

 brings me to the recent discoveries in the field of X-rays, 

 and I will call the seventh of the modern advances the 

 discovery of the nature of X-rays, which was virtually 

 made by Barkla in 1904. For Barkla and others had 

 proved that there are two types of X-rays : first, X-rays 

 which consist in simple ether pulses pushed off from an 

 electron when it changes its speed ; and second, so-called 

 characteristic X-rays. When the electrons bump into a 

 target they set something in the target into vibration, and 

 this something sends off perfectly definite characteristic 

 X-rays, which are like monochromatic light. So that we 

 have two types of X-rays, pulse X-rays, like white light ; 

 and monochromatic X-rays, like monochromatic light 

 (such as mercury gives rise to). That is the seventh of 

 our great modern discoveries, and it must be credited 

 chiefly to Barkla. 



The eighth I will call the discovery of crystal structure 

 by the study of X-rays, which is due to Laue in Munich, 



