38 Master Minds of Modern Science 



journey during which the helium atom must have passed 

 through many millions of other atoms. 



" You cannot force one billiard ball through another,' ' 

 says Sir William. " The moving ball either pushes the 

 still ball in front of it, or both move away at different 

 angles.' ' Now the helium atom could not push millions 

 of other atoms before it, yet its course was dead straight. 

 Thus it must have gone through the other atoms, and so 

 we have definite and conclusive proof that an atom is not 

 a solid body and that two atoms can occupy the same 

 space. 



Mr C. P. R. Wilson, who has worked much with Sir 

 William Bragg, emphasized this fact when he passed an 

 alpha particle (the smallest particle known) through 

 damp air, and succeeded in photographing the tiny trail 

 of mist or fog which it left behind it in its extremely 

 rapid passage. 



Sir William's chief work has been the exploration of the 

 X-ray. In 1908, when Sir William became Cavendish 

 Professor at Leeds University, the X-ray was in constant 

 use in surgery, yet there was still much doubt as to the 

 actual working of these rays. In other words, the people 

 who used the rays had very little idea as to how they 

 worked and why they penetrated solid bodies. This was 

 the task Sir William set himself ; he resolved to find out 

 exactly what happened, and in a long course of experi- 

 ments he proved that X-rays themselves do nothing to 

 the matter through which they pass. What actually hap- 

 pens is that they produce a comparatively small number 

 of fast-moving beta particles which start off at great 

 speed, and it is these electrons which do the work. 

 " They may," said Sir William, " be compared to stones 

 which on the level remain at rest, but when started down 

 hill become extremely active." 



In the course of his experiments Sir William made 

 the interesting discovery that X-rays, which had been 



