Foundations of the Universe 255 



that the mass of each particle was exceedingly small, about 

 of the mass of a hydrogen atom, the lightest atom known to 

 science." These particles or electrons, as they are now called, 

 were being liberated from the atom. The atoms of matter were 

 breaking down in Crookes tubes. At that time, however, it was 

 premature to think of such a thing, and Crookes preferred to say 

 that the particles of the gas were electrified and hurled against 

 the walls of the tube. He said that it was ordinary matter in a 

 new state "radiant matter." Another distinguished man of 

 science, Lenard, found that, when he fitted a little plate of 

 aluminum in the glass wall of the tube, the mysterious rays 

 passed through this as if it were a window. They must be waves 

 in the ether, he said. 



4 

 The Discovery of X-rays 



So the story went on from year to year. We shall see in a 

 moment to what it led. Meanwhile the next great step was when, 

 in 1895, Rontgen discovered the X-rays, which are now known 

 to everybody. He was following up the work of Lenard, and he 

 one day covered a "Crookes tube" with some black stuff. To his 

 astonishment a prepared chemical screen which was near the tube 

 began to glow. The rays had gone through the black stuff; and 

 on further experiment he found that they would go through 

 stone, living flesh, and all sorts of "opaque" substances. In a 

 short time the world was astonished to learn that we could photo- 

 graph the skeleton in a living man's body, locate a penny in the 

 interior of a child that had swallowed one, or take an impression 

 of a coin through a slab of stone. 



And what are these X-rays ? They are not a form of matter ; 

 they are not material particles. X-rays were found to be a new 

 variety of light with a remarkable power of penetration. We 

 have seen what the spectroscope reveals about the varying nature 

 of light wave-lengths. Light-waves are set up by vibrations in 



