190 THE REALITIES OF MODERN SCIENCE 



When the electrons are allowed to strike a piece of 

 platinum, as A of Fig. 19, they give rise, under proper 

 conditions of vacuum, to what have been called X-rays, 

 since their discovery by Rontgen in 1895. Their im- 

 pacts with the platinum " anti-cathode" result in very 



large changes in their 

 momenta, since they 

 b_ are moving with ve- 

 locities nearly that of 

 light, for which the 

 velocity is 3 X10 10 cm. 

 per sec. Just as the 



periodic vibrations of electrons radiate energy so the 

 sudden change in motion sends out energy in radial 

 lines. Except for the enormous difference in the velocity 

 and for the fact that the medium is different, the phe- 

 nomenon is somewhat similar to the sharp crack which 

 accompanies the blow of a baseball and a bat. There 

 travels outward through the ether a sudden pulse. When 

 this pulse strikes any substance it is so violent that it 

 affects the atoms well within the body instead of merely 

 those at the surface as does the energy of light and 

 heat waves. In other words, these pulses have extreme 

 penetration. The penetration depends upon the sub- 

 stance, and is much less for those denser substances 

 like the metals and particularly for lead. A substance 

 may partially absorb the pulses and hence cast a 

 shadow depending upon its density and thickness. 

 These X-rays therefore admit of our taking shadow 

 pictures. The shadows may be observed by inter- 

 posing the object, as for example the human body, 

 between the source and a screen of zinc sulphide, or a 



