Now Crookes really had the great secret under his eyes. But about twenty years elapsed before the true nature of these rays was finally and independently established by various experiments. The experiments proved "that the rays consisted of a stream of negatively charged particles travelling with enormous velocities from 10,000 to 100,000 miles a second. In addition, it was found that the mass of each particle was exceedingly small, about 1/1800 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. [Illustration: _From "Scientific Ideas of To-day_." DETECTING A SMALL QUANTITY OF MATTER In the left-hand photograph the two pieces of paper exactly balance. The balance used is very sensitive, and when the single word "atoms" has been written with a lead pencil upon one of the papers the additional weight is sufficient to depress one of the pans as shown in the second photograph. The spectroscope will detect less than one-millionth of the matter contained in the word pencilled above.] [Illustration: _Reproduced by permission of X-Rays Ltd._ THIS X-RAY PHOTOGRAPH IS THAT OF A HAND OF A SOLDIER WOUNDED IN THE GREAT WAR Note the pieces of shrapnel which are revealed.] [Illustration: _Photo: National Physical Laboratory._ AN X-RAY PHOTOGRAPH OF A GOLF BALL, REVEALING AN IMPERFECT CORE] [Illustration: _Reproduced by permission of X-Rays Ltd._ A WONDERFUL X-RAY PHOTOGRAPH Note the fine details revealed, down to the metal tags of the bootlace and the nails in the heel of the boot.] Sec. 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, Roentgen 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 photograph 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.